diff --git a/app/Models/Invoice.php b/app/Models/Invoice.php
new file mode 100644
index 0000000..98a6e27
--- /dev/null
+++ b/app/Models/Invoice.php
@@ -0,0 +1,61 @@
+<?php
+
+namespace App\Models;
+
+use App\Models\Concerns\HasUuid;
+use Illuminate\Database\Eloquent\Model;
+use Illuminate\Database\Eloquent\Relations\BelongsTo;
+
+/**
+ * An issued invoice, frozen.
+ *
+ * `snapshot` holds everything the document says — issuer, customer, lines, rate,
+ * totals — as it was on the day it was issued. The PDF is rendered from that and
+ * never stored, so an address changed in 2028 cannot rewrite an invoice from
+ * 2026. Nothing here is ever edited: a wrong invoice is corrected by issuing a
+ * cancellation and a new one, which is the only lawful way to correct one.
+ */
+class Invoice extends Model
+{
+    use HasUuid;
+
+    protected $fillable = [
+        'invoice_series_id', 'customer_id', 'order_id', 'number', 'number_year', 'number_sequence',
+        'issued_on', 'due_on', 'snapshot', 'net_cents', 'tax_cents', 'gross_cents', 'currency',
+        'cancels_invoice_id', 'sent_at',
+    ];
+
+    protected function casts(): array
+    {
+        return [
+            'snapshot' => 'array',
+            'issued_on' => 'date',
+            'due_on' => 'date',
+            'sent_at' => 'datetime',
+            'net_cents' => 'integer',
+            'tax_cents' => 'integer',
+            'gross_cents' => 'integer',
+        ];
+    }
+
+    public function series(): BelongsTo
+    {
+        return $this->belongsTo(InvoiceSeries::class, 'invoice_series_id');
+    }
+
+    public function customer(): BelongsTo
+    {
+        return $this->belongsTo(Customer::class);
+    }
+
+    public function order(): BelongsTo
+    {
+        return $this->belongsTo(Order::class);
+    }
+
+    /** What this document cancels, if it is a cancellation. */
+    public function cancels(): BelongsTo
+    {
+        return $this->belongsTo(Invoice::class, 'cancels_invoice_id');
+    }
+}
diff --git a/app/Models/InvoiceSeries.php b/app/Models/InvoiceSeries.php
new file mode 100644
index 0000000..526efa7
--- /dev/null
+++ b/app/Models/InvoiceSeries.php
@@ -0,0 +1,41 @@
+<?php
+
+namespace App\Models;
+
+use App\Models\Concerns\HasUuid;
+use Illuminate\Database\Eloquent\Model;
+use Illuminate\Database\Eloquent\Relations\HasMany;
+
+/**
+ * A Rechnungskreis: invoice, credit note, cancellation, deposit.
+ *
+ * Each keeps its own prefix and its own gapless counter, because they are
+ * separate number ranges in law as much as in practice — a credit note is not
+ * an invoice with a minus sign in front of it.
+ */
+class InvoiceSeries extends Model
+{
+    use HasUuid;
+
+    protected $table = 'invoice_series';
+
+    protected $fillable = [
+        'kind', 'name', 'prefix', 'yearly_reset', 'digits', 'next_number', 'counter_year', 'active',
+    ];
+
+    protected function casts(): array
+    {
+        return [
+            'yearly_reset' => 'boolean',
+            'active' => 'boolean',
+            'digits' => 'integer',
+            'next_number' => 'integer',
+            'counter_year' => 'integer',
+        ];
+    }
+
+    public function invoices(): HasMany
+    {
+        return $this->hasMany(Invoice::class);
+    }
+}
diff --git a/app/Services/Billing/InvoiceNumbers.php b/app/Services/Billing/InvoiceNumbers.php
new file mode 100644
index 0000000..148fd4e
--- /dev/null
+++ b/app/Services/Billing/InvoiceNumbers.php
@@ -0,0 +1,77 @@
+<?php
+
+namespace App\Services\Billing;
+
+use App\Models\InvoiceSeries;
+use Illuminate\Support\Facades\DB;
+
+/**
+ * Handing out invoice numbers, one at a time, without gaps.
+ *
+ * Gapless and ascending is a legal requirement in Austria, not a preference,
+ * and it is the reason none of this is derived from the invoices table. Taking
+ * "the highest number that exists plus one" hands the same number out twice the
+ * moment two sales land together, and hands a used number back out after a row
+ * is removed — a number that is already printed on a document somebody has.
+ *
+ * So the counter is a column, and it is read and advanced under a row lock
+ * inside the caller's transaction. Two concurrent sales queue; neither gets the
+ * other's number.
+ */
+final class InvoiceNumbers
+{
+    /**
+     * Take the next number in a series.
+     *
+     * MUST be called inside a transaction — the lock is worthless otherwise,
+     * and so is the invoice row that was supposed to consume the number.
+     *
+     * @return array{string, int, ?int} the formatted number, its sequence, its year
+     */
+    public function next(InvoiceSeries $series, ?int $year = null): array
+    {
+        if (! DB::transactionLevel()) {
+            throw new \LogicException(
+                'Invoice numbers must be taken inside a transaction: the number and the invoice have to commit together, or the series has a gap in it.'
+            );
+        }
+
+        $year ??= (int) now()->format('Y');
+
+        // lockForUpdate, not an atomic increment: the year check below has to
+        // see a value nobody else can change between the read and the write.
+        $locked = InvoiceSeries::query()
+            ->whereKey($series->getKey())
+            ->lockForUpdate()
+            ->firstOrFail();
+
+        $sequence = (int) $locked->next_number;
+        $counterYear = $locked->counter_year;
+
+        // A yearly series starts at 1 in a new year. Checked against the stored
+        // year rather than against "is this January": an installation that
+        // issued nothing for fourteen months must still restart, and one that
+        // issues on New Year's Eve at 23:59 must not restart for the invoice
+        // that follows a minute later under the old year.
+        if ($locked->yearly_reset && $counterYear !== null && (int) $counterYear !== $year) {
+            $sequence = 1;
+        }
+
+        $locked->forceFill([
+            'next_number' => $sequence + 1,
+            'counter_year' => $locked->yearly_reset ? $year : null,
+        ])->save();
+
+        return [$this->format($locked, $sequence, $year), $sequence, $locked->yearly_reset ? $year : null];
+    }
+
+    /** "RE-2026-0001", or "RE-0001" for a series that does not reset. */
+    private function format(InvoiceSeries $series, int $sequence, int $year): string
+    {
+        $padded = str_pad((string) $sequence, (int) $series->digits, '0', STR_PAD_LEFT);
+
+        return $series->yearly_reset
+            ? sprintf('%s-%d-%s', $series->prefix, $year, $padded)
+            : sprintf('%s-%s', $series->prefix, $padded);
+    }
+}
diff --git a/composer.json b/composer.json
index c78ddb6..2d8fee4 100644
--- a/composer.json
+++ b/composer.json
@@ -15,6 +15,7 @@
         "livewire/livewire": "^3.0",
         "phpseclib/phpseclib": "^3.0",
         "spatie/laravel-permission": "^8.3",
+        "tecnickcom/tcpdf": "^7.0",
         "wire-elements/modal": "^3.0"
     },
     "require-dev": {
diff --git a/composer.lock b/composer.lock
index c080988..65753b3 100644
--- a/composer.lock
+++ b/composer.lock
@@ -4,7 +4,7 @@
         "Read more about it at https://getcomposer.org/doc/01-basic-usage.md#installing-dependencies",
         "This file is @generated automatically"
     ],
-    "content-hash": "4afacce87fbd845b941534e375dc082b",
+    "content-hash": "4dd7de1f16d082e59e6993debfbac9b1",
     "packages": [
         {
             "name": "bacon/bacon-qr-code",
@@ -8352,6 +8352,1001 @@
             ],
             "time": "2026-06-08T20:24:16+00:00"
         },
+        {
+            "name": "tecnickcom/tc-lib-barcode",
+            "version": "2.12.0",
+            "source": {
+                "type": "git",
+                "url": "https://github.com/tecnickcom/tc-lib-barcode.git",
+                "reference": "0614e905c903600c2491d3a90174a9a87a2fab9b"
+            },
+            "dist": {
+                "type": "zip",
+                "url": "https://api.github.com/repos/tecnickcom/tc-lib-barcode/zipball/0614e905c903600c2491d3a90174a9a87a2fab9b",
+                "reference": "0614e905c903600c2491d3a90174a9a87a2fab9b",
+                "shasum": ""
+            },
+            "require": {
+                "ext-bcmath": "*",
+                "ext-gd": "*",
+                "php": ">=8.2",
+                "tecnickcom/tc-lib-color": "^2.13"
+            },
+            "require-dev": {
+                "pdepend/pdepend": "^2.16",
+                "phpunit/phpunit": "^11.5 || ^12.5 || ^13.2"
+            },
+            "type": "library",
+            "autoload": {
+                "psr-4": {
+                    "Com\\Tecnick\\Barcode\\": "src"
+                }
+            },
+            "notification-url": "https://packagist.org/downloads/",
+            "license": [
+                "LGPL-3.0-or-later"
+            ],
+            "authors": [
+                {
+                    "name": "Nicola Asuni",
+                    "email": "info@tecnick.com",
+                    "role": "lead"
+                }
+            ],
+            "description": "PHP library to generate linear and bidimensional barcodes",
+            "homepage": "https://tcpdf.org",
+            "keywords": [
+                "3 of 9",
+                "ANSI MH10.8M-1983",
+                "CBC",
+                "CODABAR",
+                "CODE 11",
+                "CODE 128 A B C",
+                "CODE 39",
+                "CODE 93",
+                "EAN 13",
+                "EAN 8",
+                "ECC200",
+                "ISO IEC 15438 2006",
+                "ISO IEC 16022",
+                "ISO IEC 24778 2008",
+                "Intelligent Mail Barcode",
+                "Interleaved 2 of 5",
+                "KIX",
+                "Klant",
+                "MSI",
+                "Onecode",
+                "PHARMACODE",
+                "PHARMACODE TWO-TRACKS",
+                "POSTNET",
+                "RMS4CC",
+                "Standard 2 of 5",
+                "UPC-A",
+                "UPC-E",
+                "USD-3",
+                "USPS-B-3200",
+                "USS-93",
+                "aztec",
+                "barcode",
+                "datamatrix",
+                "pdf417",
+                "planet",
+                "qr-code",
+                "royal mail",
+                "tc-lib-barcode",
+                "upc"
+            ],
+            "support": {
+                "issues": "https://github.com/tecnickcom/tc-lib-barcode/issues",
+                "source": "https://github.com/tecnickcom/tc-lib-barcode"
+            },
+            "funding": [
+                {
+                    "url": "https://github.com/sponsors/tecnickcom",
+                    "type": "github"
+                }
+            ],
+            "time": "2026-07-17T19:05:47+00:00"
+        },
+        {
+            "name": "tecnickcom/tc-lib-color",
+            "version": "2.13.1",
+            "source": {
+                "type": "git",
+                "url": "https://github.com/tecnickcom/tc-lib-color.git",
+                "reference": "38303066464928bbcfe8e54d006c828b02e4c62f"
+            },
+            "dist": {
+                "type": "zip",
+                "url": "https://api.github.com/repos/tecnickcom/tc-lib-color/zipball/38303066464928bbcfe8e54d006c828b02e4c62f",
+                "reference": "38303066464928bbcfe8e54d006c828b02e4c62f",
+                "shasum": ""
+            },
+            "require": {
+                "php": ">=8.2"
+            },
+            "require-dev": {
+                "pdepend/pdepend": "^2.16",
+                "phpunit/phpunit": "^11.5 || ^12.5 || ^13.2"
+            },
+            "type": "library",
+            "autoload": {
+                "psr-4": {
+                    "Com\\Tecnick\\Color\\": "src"
+                }
+            },
+            "notification-url": "https://packagist.org/downloads/",
+            "license": [
+                "LGPL-3.0-or-later"
+            ],
+            "authors": [
+                {
+                    "name": "Nicola Asuni",
+                    "email": "info@tecnick.com",
+                    "role": "lead"
+                }
+            ],
+            "description": "PHP library to manipulate various color representations",
+            "homepage": "https://tcpdf.org",
+            "keywords": [
+                "cmyk",
+                "color",
+                "colors",
+                "colour",
+                "colours",
+                "hsl",
+                "hsla",
+                "javascript",
+                "rgb",
+                "rgba",
+                "tc-lib-color",
+                "web"
+            ],
+            "support": {
+                "issues": "https://github.com/tecnickcom/tc-lib-color/issues",
+                "source": "https://github.com/tecnickcom/tc-lib-color"
+            },
+            "funding": [
+                {
+                    "url": "https://github.com/sponsors/tecnickcom",
+                    "type": "github"
+                }
+            ],
+            "time": "2026-07-17T19:02:53+00:00"
+        },
+        {
+            "name": "tecnickcom/tc-lib-file",
+            "version": "3.7.1",
+            "source": {
+                "type": "git",
+                "url": "https://github.com/tecnickcom/tc-lib-file.git",
+                "reference": "45a9b04e530b80b0212af6de1c6c111c37f8eb35"
+            },
+            "dist": {
+                "type": "zip",
+                "url": "https://api.github.com/repos/tecnickcom/tc-lib-file/zipball/45a9b04e530b80b0212af6de1c6c111c37f8eb35",
+                "reference": "45a9b04e530b80b0212af6de1c6c111c37f8eb35",
+                "shasum": ""
+            },
+            "require": {
+                "ext-curl": "*",
+                "php": ">=8.2"
+            },
+            "require-dev": {
+                "pdepend/pdepend": "^2.16",
+                "phpunit/phpunit": "^13.2 || ^12.5 || ^11.5"
+            },
+            "type": "library",
+            "autoload": {
+                "psr-4": {
+                    "Com\\Tecnick\\File\\": "src"
+                }
+            },
+            "notification-url": "https://packagist.org/downloads/",
+            "license": [
+                "LGPL-3.0-or-later"
+            ],
+            "authors": [
+                {
+                    "name": "Nicola Asuni",
+                    "email": "info@tecnick.com",
+                    "role": "lead"
+                }
+            ],
+            "description": "PHP library to read byte-level data from files",
+            "homepage": "https://tcpdf.org",
+            "keywords": [
+                "Double",
+                "bit",
+                "byte",
+                "file",
+                "long",
+                "read",
+                "short",
+                "tc-lib-file"
+            ],
+            "support": {
+                "issues": "https://github.com/tecnickcom/tc-lib-file/issues",
+                "source": "https://github.com/tecnickcom/tc-lib-file"
+            },
+            "funding": [
+                {
+                    "url": "https://github.com/sponsors/tecnickcom",
+                    "type": "github"
+                }
+            ],
+            "time": "2026-07-17T19:03:05+00:00"
+        },
+        {
+            "name": "tecnickcom/tc-lib-pdf",
+            "version": "8.67.2",
+            "source": {
+                "type": "git",
+                "url": "https://github.com/tecnickcom/tc-lib-pdf.git",
+                "reference": "f001da787064da712104f88250db6a6af66929f4"
+            },
+            "dist": {
+                "type": "zip",
+                "url": "https://api.github.com/repos/tecnickcom/tc-lib-pdf/zipball/f001da787064da712104f88250db6a6af66929f4",
+                "reference": "f001da787064da712104f88250db6a6af66929f4",
+                "shasum": ""
+            },
+            "require": {
+                "php": ">=8.2",
+                "tecnickcom/tc-lib-barcode": "^2.12",
+                "tecnickcom/tc-lib-color": "^2.13",
+                "tecnickcom/tc-lib-file": "^3.7",
+                "tecnickcom/tc-lib-pdf-encrypt": "^2.9",
+                "tecnickcom/tc-lib-pdf-font": "^3.12",
+                "tecnickcom/tc-lib-pdf-graph": "^2.15",
+                "tecnickcom/tc-lib-pdf-image": "^3.12",
+                "tecnickcom/tc-lib-pdf-page": "^4.14",
+                "tecnickcom/tc-lib-pdf-parser": "^3.14",
+                "tecnickcom/tc-lib-pdf-sign": "^1.1",
+                "tecnickcom/tc-lib-unicode": "^2.11",
+                "tecnickcom/tc-lib-unicode-data": "^2.7"
+            },
+            "require-dev": {
+                "pdepend/pdepend": "^2.16",
+                "phpunit/phpunit": "^11.5 || ^12.5 || ^13.2"
+            },
+            "type": "library",
+            "autoload": {
+                "psr-4": {
+                    "Com\\Tecnick\\Pdf\\": "src"
+                }
+            },
+            "notification-url": "https://packagist.org/downloads/",
+            "license": [
+                "LGPL-3.0-or-later"
+            ],
+            "authors": [
+                {
+                    "name": "Nicola Asuni",
+                    "email": "info@tecnick.com",
+                    "role": "lead"
+                }
+            ],
+            "description": "PHP PDF Library",
+            "homepage": "https://tcpdf.org",
+            "keywords": [
+                "PDFD32000-2008",
+                "TCPDF",
+                "document",
+                "pdf",
+                "tc-lib-pdf"
+            ],
+            "support": {
+                "issues": "https://github.com/tecnickcom/tc-lib-pdf/issues",
+                "source": "https://github.com/tecnickcom/tc-lib-pdf"
+            },
+            "funding": [
+                {
+                    "url": "https://github.com/sponsors/tecnickcom",
+                    "type": "github"
+                }
+            ],
+            "time": "2026-07-22T12:58:58+00:00"
+        },
+        {
+            "name": "tecnickcom/tc-lib-pdf-encrypt",
+            "version": "2.9.1",
+            "source": {
+                "type": "git",
+                "url": "https://github.com/tecnickcom/tc-lib-pdf-encrypt.git",
+                "reference": "c13ae45c9ac5be8c1974c43727879411f0c557e4"
+            },
+            "dist": {
+                "type": "zip",
+                "url": "https://api.github.com/repos/tecnickcom/tc-lib-pdf-encrypt/zipball/c13ae45c9ac5be8c1974c43727879411f0c557e4",
+                "reference": "c13ae45c9ac5be8c1974c43727879411f0c557e4",
+                "shasum": ""
+            },
+            "require": {
+                "ext-hash": "*",
+                "ext-openssl": "*",
+                "php": ">=8.2"
+            },
+            "require-dev": {
+                "pdepend/pdepend": "^2.16",
+                "phpunit/phpunit": "^11.5 || ^12.5 || ^13.2"
+            },
+            "suggest": {
+                "ext-posix": "*"
+            },
+            "type": "library",
+            "autoload": {
+                "psr-4": {
+                    "Com\\Tecnick\\Pdf\\Encrypt\\": "src"
+                }
+            },
+            "notification-url": "https://packagist.org/downloads/",
+            "license": [
+                "LGPL-3.0-or-later"
+            ],
+            "authors": [
+                {
+                    "name": "Nicola Asuni",
+                    "email": "info@tecnick.com",
+                    "role": "lead"
+                }
+            ],
+            "description": "PHP library to encrypt data for PDF",
+            "homepage": "https://tcpdf.org",
+            "keywords": [
+                "aes",
+                "encrypt",
+                "encryption",
+                "pdf",
+                "rc4",
+                "tc-lib-pdf-encrypt"
+            ],
+            "support": {
+                "issues": "https://github.com/tecnickcom/tc-lib-pdf-encrypt/issues",
+                "source": "https://github.com/tecnickcom/tc-lib-pdf-encrypt"
+            },
+            "funding": [
+                {
+                    "url": "https://github.com/sponsors/tecnickcom",
+                    "type": "github"
+                }
+            ],
+            "time": "2026-07-17T19:03:18+00:00"
+        },
+        {
+            "name": "tecnickcom/tc-lib-pdf-filter",
+            "version": "2.10.1",
+            "source": {
+                "type": "git",
+                "url": "https://github.com/tecnickcom/tc-lib-pdf-filter.git",
+                "reference": "72d7606dbc56f475870222557488edec3d707e8c"
+            },
+            "dist": {
+                "type": "zip",
+                "url": "https://api.github.com/repos/tecnickcom/tc-lib-pdf-filter/zipball/72d7606dbc56f475870222557488edec3d707e8c",
+                "reference": "72d7606dbc56f475870222557488edec3d707e8c",
+                "shasum": ""
+            },
+            "require": {
+                "ext-zlib": "*",
+                "php": ">=8.2"
+            },
+            "require-dev": {
+                "pdepend/pdepend": "^2.16",
+                "phpunit/phpunit": "^11.5 || ^12.5 || ^13.2"
+            },
+            "suggest": {
+                "ext-imagick": "Required for JPXDecode (JPEG 2000) filter support"
+            },
+            "type": "library",
+            "autoload": {
+                "psr-4": {
+                    "Com\\Tecnick\\Pdf\\Filter\\": "src"
+                }
+            },
+            "notification-url": "https://packagist.org/downloads/",
+            "license": [
+                "LGPL-3.0-or-later"
+            ],
+            "authors": [
+                {
+                    "name": "Nicola Asuni",
+                    "email": "info@tecnick.com",
+                    "role": "lead"
+                }
+            ],
+            "description": "PHP library to decode PDF compression and encryption filters",
+            "homepage": "https://tcpdf.org",
+            "keywords": [
+                "compression",
+                "encryption",
+                "filter",
+                "pdf",
+                "tc-lib-pdf-filter"
+            ],
+            "support": {
+                "issues": "https://github.com/tecnickcom/tc-lib-pdf-filter/issues",
+                "source": "https://github.com/tecnickcom/tc-lib-pdf-filter"
+            },
+            "funding": [
+                {
+                    "url": "https://github.com/sponsors/tecnickcom",
+                    "type": "github"
+                }
+            ],
+            "time": "2026-07-17T19:03:30+00:00"
+        },
+        {
+            "name": "tecnickcom/tc-lib-pdf-font",
+            "version": "3.12.0",
+            "source": {
+                "type": "git",
+                "url": "https://github.com/tecnickcom/tc-lib-pdf-font.git",
+                "reference": "e84e765474d15f2d7f19f5ff8f42604c69a3a41a"
+            },
+            "dist": {
+                "type": "zip",
+                "url": "https://api.github.com/repos/tecnickcom/tc-lib-pdf-font/zipball/e84e765474d15f2d7f19f5ff8f42604c69a3a41a",
+                "reference": "e84e765474d15f2d7f19f5ff8f42604c69a3a41a",
+                "shasum": ""
+            },
+            "require": {
+                "ext-json": "*",
+                "ext-zlib": "*",
+                "php": ">=8.2",
+                "tecnickcom/tc-lib-file": "^3.7",
+                "tecnickcom/tc-lib-pdf-encrypt": "^2.9",
+                "tecnickcom/tc-lib-unicode-data": "^2.7"
+            },
+            "require-dev": {
+                "pdepend/pdepend": "^2.16",
+                "phpunit/phpunit": "^11.5 || ^12.5 || ^13.2"
+            },
+            "type": "library",
+            "autoload": {
+                "psr-4": {
+                    "Com\\Tecnick\\Pdf\\Font\\": "src"
+                }
+            },
+            "notification-url": "https://packagist.org/downloads/",
+            "license": [
+                "LGPL-3.0-or-later"
+            ],
+            "authors": [
+                {
+                    "name": "Nicola Asuni",
+                    "email": "info@tecnick.com",
+                    "role": "lead"
+                }
+            ],
+            "description": "PHP library containing PDF page formats and definitions",
+            "homepage": "https://tcpdf.org",
+            "keywords": [
+                "PFB",
+                "afm",
+                "font",
+                "import",
+                "pdf",
+                "tc-lib-pdf-font",
+                "ttf"
+            ],
+            "support": {
+                "issues": "https://github.com/tecnickcom/tc-lib-pdf-font/issues",
+                "source": "https://github.com/tecnickcom/tc-lib-pdf-font"
+            },
+            "funding": [
+                {
+                    "url": "https://github.com/sponsors/tecnickcom",
+                    "type": "github"
+                }
+            ],
+            "time": "2026-07-17T19:22:14+00:00"
+        },
+        {
+            "name": "tecnickcom/tc-lib-pdf-graph",
+            "version": "2.15.0",
+            "source": {
+                "type": "git",
+                "url": "https://github.com/tecnickcom/tc-lib-pdf-graph.git",
+                "reference": "cdd4ee1ceafc0f2cfa525ef8f395c80e7c024495"
+            },
+            "dist": {
+                "type": "zip",
+                "url": "https://api.github.com/repos/tecnickcom/tc-lib-pdf-graph/zipball/cdd4ee1ceafc0f2cfa525ef8f395c80e7c024495",
+                "reference": "cdd4ee1ceafc0f2cfa525ef8f395c80e7c024495",
+                "shasum": ""
+            },
+            "require": {
+                "ext-zlib": "*",
+                "php": ">=8.2",
+                "tecnickcom/tc-lib-color": "^2.13",
+                "tecnickcom/tc-lib-pdf-encrypt": "^2.9"
+            },
+            "require-dev": {
+                "pdepend/pdepend": "^2.16",
+                "phpunit/phpunit": "^11.5 || ^12.5 || ^13.2"
+            },
+            "type": "library",
+            "autoload": {
+                "psr-4": {
+                    "Com\\Tecnick\\Pdf\\Graph\\": "src"
+                }
+            },
+            "notification-url": "https://packagist.org/downloads/",
+            "license": [
+                "LGPL-3.0-or-later"
+            ],
+            "authors": [
+                {
+                    "name": "Nicola Asuni",
+                    "email": "info@tecnick.com",
+                    "role": "lead"
+                }
+            ],
+            "description": "PHP library containing PDF graphic and geometric methods",
+            "homepage": "https://tcpdf.org",
+            "keywords": [
+                "geometry",
+                "graphic",
+                "pdf",
+                "tc-lib-pdf-graph",
+                "transformation"
+            ],
+            "support": {
+                "issues": "https://github.com/tecnickcom/tc-lib-pdf-graph/issues",
+                "source": "https://github.com/tecnickcom/tc-lib-pdf-graph"
+            },
+            "funding": [
+                {
+                    "url": "https://github.com/sponsors/tecnickcom",
+                    "type": "github"
+                }
+            ],
+            "time": "2026-07-17T19:18:50+00:00"
+        },
+        {
+            "name": "tecnickcom/tc-lib-pdf-image",
+            "version": "3.12.0",
+            "source": {
+                "type": "git",
+                "url": "https://github.com/tecnickcom/tc-lib-pdf-image.git",
+                "reference": "d20db0e8e371ba1a44c2dc8358320599360be7f1"
+            },
+            "dist": {
+                "type": "zip",
+                "url": "https://api.github.com/repos/tecnickcom/tc-lib-pdf-image/zipball/d20db0e8e371ba1a44c2dc8358320599360be7f1",
+                "reference": "d20db0e8e371ba1a44c2dc8358320599360be7f1",
+                "shasum": ""
+            },
+            "require": {
+                "ext-gd": "*",
+                "ext-zlib": "*",
+                "php": ">=8.2",
+                "tecnickcom/tc-lib-color": "^2.13",
+                "tecnickcom/tc-lib-file": "^3.7",
+                "tecnickcom/tc-lib-pdf-encrypt": "^2.9"
+            },
+            "require-dev": {
+                "pdepend/pdepend": "^2.16",
+                "phpunit/phpunit": "^11.5 || ^12.5 || ^13.2"
+            },
+            "type": "library",
+            "autoload": {
+                "psr-4": {
+                    "Com\\Tecnick\\Pdf\\Image\\": "src"
+                }
+            },
+            "notification-url": "https://packagist.org/downloads/",
+            "license": [
+                "LGPL-3.0-or-later"
+            ],
+            "authors": [
+                {
+                    "name": "Nicola Asuni",
+                    "email": "info@tecnick.com",
+                    "role": "lead"
+                }
+            ],
+            "description": "PHP library containing PDF Image methods",
+            "homepage": "https://tcpdf.org",
+            "keywords": [
+                "image",
+                "jpeg",
+                "jpg",
+                "pdf",
+                "png",
+                "tc-lib-pdf-image"
+            ],
+            "support": {
+                "issues": "https://github.com/tecnickcom/tc-lib-pdf-image/issues",
+                "source": "https://github.com/tecnickcom/tc-lib-pdf-image"
+            },
+            "funding": [
+                {
+                    "url": "https://github.com/sponsors/tecnickcom",
+                    "type": "github"
+                }
+            ],
+            "time": "2026-07-17T19:20:33+00:00"
+        },
+        {
+            "name": "tecnickcom/tc-lib-pdf-page",
+            "version": "4.14.0",
+            "source": {
+                "type": "git",
+                "url": "https://github.com/tecnickcom/tc-lib-pdf-page.git",
+                "reference": "a068a4b2da222bd9909bb1e9edeec901a69a6484"
+            },
+            "dist": {
+                "type": "zip",
+                "url": "https://api.github.com/repos/tecnickcom/tc-lib-pdf-page/zipball/a068a4b2da222bd9909bb1e9edeec901a69a6484",
+                "reference": "a068a4b2da222bd9909bb1e9edeec901a69a6484",
+                "shasum": ""
+            },
+            "require": {
+                "ext-zlib": "*",
+                "php": ">=8.2",
+                "tecnickcom/tc-lib-color": "^2.13",
+                "tecnickcom/tc-lib-pdf-encrypt": "^2.9"
+            },
+            "require-dev": {
+                "pdepend/pdepend": "^2.16",
+                "phpunit/phpunit": "^11.5 || ^12.5 || ^13.2"
+            },
+            "type": "library",
+            "autoload": {
+                "psr-4": {
+                    "Com\\Tecnick\\Pdf\\Page\\": "src"
+                }
+            },
+            "notification-url": "https://packagist.org/downloads/",
+            "license": [
+                "LGPL-3.0-or-later"
+            ],
+            "authors": [
+                {
+                    "name": "Nicola Asuni",
+                    "email": "info@tecnick.com",
+                    "role": "lead"
+                }
+            ],
+            "description": "PHP library containing PDF page formats and definitions",
+            "homepage": "https://tcpdf.org",
+            "keywords": [
+                "format",
+                "page",
+                "pdf",
+                "tc-lib-pdf-page"
+            ],
+            "support": {
+                "issues": "https://github.com/tecnickcom/tc-lib-pdf-page/issues",
+                "source": "https://github.com/tecnickcom/tc-lib-pdf-page"
+            },
+            "funding": [
+                {
+                    "url": "https://github.com/sponsors/tecnickcom",
+                    "type": "github"
+                }
+            ],
+            "time": "2026-07-17T19:17:39+00:00"
+        },
+        {
+            "name": "tecnickcom/tc-lib-pdf-parser",
+            "version": "3.14.0",
+            "source": {
+                "type": "git",
+                "url": "https://github.com/tecnickcom/tc-lib-pdf-parser.git",
+                "reference": "8a1b64ba7a8f8c7d862765ad2fcd1841982ab5bb"
+            },
+            "dist": {
+                "type": "zip",
+                "url": "https://api.github.com/repos/tecnickcom/tc-lib-pdf-parser/zipball/8a1b64ba7a8f8c7d862765ad2fcd1841982ab5bb",
+                "reference": "8a1b64ba7a8f8c7d862765ad2fcd1841982ab5bb",
+                "shasum": ""
+            },
+            "require": {
+                "php": ">=8.2",
+                "tecnickcom/tc-lib-pdf-filter": "^2.10"
+            },
+            "require-dev": {
+                "pdepend/pdepend": "^2.16",
+                "phpunit/phpunit": "^11.5 || ^12.5 || ^13.2"
+            },
+            "type": "library",
+            "autoload": {
+                "psr-4": {
+                    "Com\\Tecnick\\Pdf\\Parser\\": "src"
+                }
+            },
+            "notification-url": "https://packagist.org/downloads/",
+            "license": [
+                "LGPL-3.0-or-later"
+            ],
+            "authors": [
+                {
+                    "name": "Nicola Asuni",
+                    "email": "info@tecnick.com",
+                    "role": "lead"
+                }
+            ],
+            "description": "PHP library to parse PDF documents",
+            "homepage": "https://tcpdf.org",
+            "keywords": [
+                "document",
+                "parser",
+                "pdf",
+                "tc-lib-pdf-parser"
+            ],
+            "support": {
+                "issues": "https://github.com/tecnickcom/tc-lib-pdf-parser/issues",
+                "source": "https://github.com/tecnickcom/tc-lib-pdf-parser"
+            },
+            "funding": [
+                {
+                    "url": "https://github.com/sponsors/tecnickcom",
+                    "type": "github"
+                }
+            ],
+            "time": "2026-07-17T19:15:54+00:00"
+        },
+        {
+            "name": "tecnickcom/tc-lib-pdf-sign",
+            "version": "1.1.1",
+            "source": {
+                "type": "git",
+                "url": "https://github.com/tecnickcom/tc-lib-pdf-sign.git",
+                "reference": "c9d5125cdcfe6225951ce2bc165f46b4d0f79bc5"
+            },
+            "dist": {
+                "type": "zip",
+                "url": "https://api.github.com/repos/tecnickcom/tc-lib-pdf-sign/zipball/c9d5125cdcfe6225951ce2bc165f46b4d0f79bc5",
+                "reference": "c9d5125cdcfe6225951ce2bc165f46b4d0f79bc5",
+                "shasum": ""
+            },
+            "require": {
+                "ext-hash": "*",
+                "ext-openssl": "*",
+                "php": ">=8.2"
+            },
+            "require-dev": {
+                "pdepend/pdepend": "^2.16",
+                "phpunit/phpunit": "^11.5 || ^12.5 || ^13.2"
+            },
+            "suggest": {
+                "ext-posix": "*"
+            },
+            "type": "library",
+            "autoload": {
+                "psr-4": {
+                    "Com\\Tecnick\\Pdf\\Sign\\": "src"
+                }
+            },
+            "notification-url": "https://packagist.org/downloads/",
+            "license": [
+                "LGPL-3.0-or-later"
+            ],
+            "authors": [
+                {
+                    "name": "Nicola Asuni",
+                    "email": "info@tecnick.com",
+                    "role": "lead"
+                }
+            ],
+            "description": "PHP library to create and embed digital signatures (PKCS#7 / CAdES / PAdES) in PDF documents",
+            "homepage": "https://tcpdf.org",
+            "keywords": [
+                "cades",
+                "digital-signature",
+                "pades",
+                "pdf",
+                "pkcs7",
+                "sign",
+                "signature",
+                "tc-lib-pdf-sign",
+                "timestamp"
+            ],
+            "support": {
+                "issues": "https://github.com/tecnickcom/tc-lib-pdf-sign/issues",
+                "source": "https://github.com/tecnickcom/tc-lib-pdf-sign"
+            },
+            "funding": [
+                {
+                    "url": "https://github.com/sponsors/tecnickcom",
+                    "type": "github"
+                }
+            ],
+            "time": "2026-07-17T19:03:43+00:00"
+        },
+        {
+            "name": "tecnickcom/tc-lib-unicode",
+            "version": "2.11.0",
+            "source": {
+                "type": "git",
+                "url": "https://github.com/tecnickcom/tc-lib-unicode.git",
+                "reference": "ffc77054b8ea2b7de8cd3fd1c02ce8fd10bb17b5"
+            },
+            "dist": {
+                "type": "zip",
+                "url": "https://api.github.com/repos/tecnickcom/tc-lib-unicode/zipball/ffc77054b8ea2b7de8cd3fd1c02ce8fd10bb17b5",
+                "reference": "ffc77054b8ea2b7de8cd3fd1c02ce8fd10bb17b5",
+                "shasum": ""
+            },
+            "require": {
+                "ext-mbstring": "*",
+                "php": ">=8.2",
+                "tecnickcom/tc-lib-unicode-data": "^2.7"
+            },
+            "require-dev": {
+                "pdepend/pdepend": "^2.16",
+                "phpunit/phpunit": "^11.5 || ^12.5 || ^13.2"
+            },
+            "type": "library",
+            "autoload": {
+                "psr-4": {
+                    "Com\\Tecnick\\Unicode\\": "src"
+                }
+            },
+            "notification-url": "https://packagist.org/downloads/",
+            "license": [
+                "LGPL-3.0-or-later"
+            ],
+            "authors": [
+                {
+                    "name": "Nicola Asuni",
+                    "email": "info@tecnick.com",
+                    "role": "lead"
+                }
+            ],
+            "description": "PHP library containing Unicode methods",
+            "homepage": "https://tcpdf.org",
+            "keywords": [
+                "font",
+                "pdf",
+                "tc-lib-unicode",
+                "unicode",
+                "utf-8"
+            ],
+            "support": {
+                "issues": "https://github.com/tecnickcom/tc-lib-unicode/issues",
+                "source": "https://github.com/tecnickcom/tc-lib-unicode"
+            },
+            "funding": [
+                {
+                    "url": "https://github.com/sponsors/tecnickcom",
+                    "type": "github"
+                }
+            ],
+            "time": "2026-07-17T19:09:36+00:00"
+        },
+        {
+            "name": "tecnickcom/tc-lib-unicode-data",
+            "version": "2.7.1",
+            "source": {
+                "type": "git",
+                "url": "https://github.com/tecnickcom/tc-lib-unicode-data.git",
+                "reference": "9354f1661e7bafcf2ee68e408ed07a3fcf264b98"
+            },
+            "dist": {
+                "type": "zip",
+                "url": "https://api.github.com/repos/tecnickcom/tc-lib-unicode-data/zipball/9354f1661e7bafcf2ee68e408ed07a3fcf264b98",
+                "reference": "9354f1661e7bafcf2ee68e408ed07a3fcf264b98",
+                "shasum": ""
+            },
+            "require": {
+                "php": ">=8.2"
+            },
+            "require-dev": {
+                "pdepend/pdepend": "^2.16",
+                "phpunit/phpunit": "^11.5 || ^12.5 || ^13.2"
+            },
+            "type": "library",
+            "autoload": {
+                "psr-4": {
+                    "Com\\Tecnick\\Unicode\\Data\\": "src"
+                }
+            },
+            "notification-url": "https://packagist.org/downloads/",
+            "license": [
+                "LGPL-3.0-or-later"
+            ],
+            "authors": [
+                {
+                    "name": "Nicola Asuni",
+                    "email": "info@tecnick.com",
+                    "role": "lead"
+                }
+            ],
+            "description": "PHP library containing Unicode definitions",
+            "homepage": "https://tcpdf.org",
+            "keywords": [
+                "font",
+                "pdf",
+                "tc-lib-unicode-data",
+                "unicode",
+                "utf-8"
+            ],
+            "support": {
+                "issues": "https://github.com/tecnickcom/tc-lib-unicode-data/issues",
+                "source": "https://github.com/tecnickcom/tc-lib-unicode-data"
+            },
+            "funding": [
+                {
+                    "url": "https://github.com/sponsors/tecnickcom",
+                    "type": "github"
+                }
+            ],
+            "time": "2026-07-17T19:03:56+00:00"
+        },
+        {
+            "name": "tecnickcom/tcpdf",
+            "version": "7.0.4",
+            "source": {
+                "type": "git",
+                "url": "https://github.com/tecnickcom/TCPDF.git",
+                "reference": "7b8de35a6224b7791a91fc09427a18a0a8aecd83"
+            },
+            "dist": {
+                "type": "zip",
+                "url": "https://api.github.com/repos/tecnickcom/TCPDF/zipball/7b8de35a6224b7791a91fc09427a18a0a8aecd83",
+                "reference": "7b8de35a6224b7791a91fc09427a18a0a8aecd83",
+                "shasum": ""
+            },
+            "require": {
+                "ext-curl": "*",
+                "php": ">=8.2",
+                "tecnickcom/tc-lib-pdf": "^8"
+            },
+            "require-dev": {
+                "pdepend/pdepend": "^2.16",
+                "phpunit/phpunit": "^11.5 || ^12.5 || ^13.2"
+            },
+            "suggest": {
+                "ext-gd": "Enables additional image handling in some workflows.",
+                "ext-imagick": "Enables additional image format support when available.",
+                "ext-zlib": "Recommended for compressed streams and related features.",
+                "tecnickcom/tc-lib-pdf": "Modern replacement for TCPDF for new projects."
+            },
+            "type": "library",
+            "autoload": {
+                "classmap": [
+                    "config",
+                    "tcpdf.php"
+                ]
+            },
+            "notification-url": "https://packagist.org/downloads/",
+            "license": [
+                "LGPL-3.0-or-later"
+            ],
+            "authors": [
+                {
+                    "name": "Nicola Asuni",
+                    "email": "info@tecnick.com",
+                    "role": "lead"
+                }
+            ],
+            "description": "Deprecated legacy PDF engine for PHP. Use instead tecnickcom/tc-lib-pdf.",
+            "homepage": "https://tcpdf.org",
+            "keywords": [
+                "PDFD32000-2008",
+                "TCPDF",
+                "barcodes",
+                "datamatrix",
+                "pdf",
+                "pdf417",
+                "qrcode"
+            ],
+            "support": {
+                "issues": "https://github.com/tecnickcom/TCPDF/issues",
+                "source": "https://github.com/tecnickcom/TCPDF"
+            },
+            "funding": [
+                {
+                    "url": "https://github.com/sponsors/tecnickcom",
+                    "type": "github"
+                }
+            ],
+            "time": "2026-07-13T10:06:03+00:00"
+        },
         {
             "name": "tijsverkoyen/css-to-inline-styles",
             "version": "v2.4.0",
diff --git a/database/migrations/2026_07_29_130000_create_invoice_tables.php b/database/migrations/2026_07_29_130000_create_invoice_tables.php
new file mode 100644
index 0000000..1f415da
--- /dev/null
+++ b/database/migrations/2026_07_29_130000_create_invoice_tables.php
@@ -0,0 +1,135 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\DB;
+use Illuminate\Support\Facades\Schema;
+use Illuminate\Support\Str;
+
+/**
+ * Invoices, frozen at the moment they are issued.
+ *
+ * The PDF is never stored. It is rendered on demand — but never from today's
+ * settings. Everything a finished invoice says is copied into `snapshot` when
+ * the number is assigned: the issuer's address, its VAT number and bank
+ * details, the customer's address, every line, the rate and the totals. Change
+ * the company address in 2028 and an invoice from 2026 still shows the address
+ * it was issued under, because that is what a tax document is.
+ *
+ * Rendering from live settings would have been simpler and wrong. It is the
+ * reason this table is wide: the width IS the feature.
+ *
+ * Numbers come from `invoice_series` — one row per Rechnungskreis (invoice,
+ * credit note, cancellation, deposit), each with its own prefix and its own
+ * gapless counter. Gapless is a legal requirement, not a preference, which is
+ * why the counter lives in the database under a row lock rather than being
+ * derived from whatever rows happen to exist.
+ */
+return new class extends Migration
+{
+    public function up(): void
+    {
+        Schema::create('invoice_series', function (Blueprint $table) {
+            $table->id();
+            $table->uuid()->unique();
+
+            // 'invoice' | 'credit_note' | 'cancellation' | 'deposit'. A string
+            // rather than an enum column: adding a kind must not be a migration
+            // on a table that holds legal records.
+            $table->string('kind', 32);
+            $table->string('name');
+
+            // "RE", "GS", "ST" — what the number starts with.
+            $table->string('prefix', 12);
+
+            // The year is part of the number and resets the counter, or it is
+            // not and does not. Both are lawful; mixing them is not, so it is
+            // recorded per series rather than assumed.
+            $table->boolean('yearly_reset')->default(true);
+            $table->unsignedSmallInteger('digits')->default(4);
+
+            // The counter itself. Never derived from the invoices table: a
+            // deleted or failed row would silently reuse a number that has
+            // already been on a document somebody has.
+            $table->unsignedInteger('next_number')->default(1);
+            $table->unsignedSmallInteger('counter_year')->nullable();
+
+            $table->boolean('active')->default(true);
+            $table->timestamps();
+
+            $table->unique(['kind', 'prefix']);
+        });
+
+        Schema::create('invoices', function (Blueprint $table) {
+            $table->id();
+            $table->uuid()->unique();
+
+            $table->foreignId('invoice_series_id')->constrained()->restrictOnDelete();
+            $table->foreignId('customer_id')->nullable()->constrained()->nullOnDelete();
+            $table->foreignId('order_id')->nullable()->constrained()->nullOnDelete();
+
+            // The number as it appears on the document, and its parts, so a
+            // list can be sorted and filtered without parsing the string back.
+            $table->string('number')->unique();
+            $table->unsignedSmallInteger('number_year')->nullable();
+            $table->unsignedInteger('number_sequence');
+
+            $table->date('issued_on');
+            $table->date('due_on')->nullable();
+
+            // Everything the document says. See the class comment: this is what
+            // makes the PDF reproducible without storing the PDF.
+            $table->json('snapshot');
+
+            // Denormalised out of the snapshot, because a list of invoices has
+            // to be sortable by money without opening every JSON column.
+            $table->integer('net_cents');
+            $table->integer('tax_cents');
+            $table->integer('gross_cents');
+            $table->string('currency', 3);
+
+            // A cancellation points at what it cancels. Nothing is ever
+            // deleted or edited — a wrong invoice is corrected by issuing
+            // another document, which is the only lawful way to correct one.
+            $table->foreignId('cancels_invoice_id')->nullable()->constrained('invoices')->nullOnDelete();
+
+            $table->timestamp('sent_at')->nullable();
+            $table->timestamps();
+
+            $table->index(['customer_id', 'issued_on']);
+        });
+
+        // The four an Austrian business needs on day one. Seeded rather than
+        // left to the operator: an installation with no series cannot issue an
+        // invoice at all, and discovering that at the moment of the first sale
+        // is the worst possible time.
+        $now = now();
+
+        foreach ([
+            ['invoice', 'Rechnung', 'RE'],
+            ['credit_note', 'Gutschrift', 'GS'],
+            ['cancellation', 'Storno', 'ST'],
+            ['deposit', 'Anzahlung', 'AZ'],
+        ] as [$kind, $name, $prefix]) {
+            DB::table('invoice_series')->insert([
+                'uuid' => (string) Str::uuid(),
+                'kind' => $kind,
+                'name' => $name,
+                'prefix' => $prefix,
+                'yearly_reset' => true,
+                'digits' => 4,
+                'next_number' => 1,
+                'counter_year' => null,
+                'active' => true,
+                'created_at' => $now,
+                'updated_at' => $now,
+            ]);
+        }
+    }
+
+    public function down(): void
+    {
+        Schema::dropIfExists('invoices');
+        Schema::dropIfExists('invoice_series');
+    }
+};
diff --git a/tests/Feature/Billing/InvoiceNumberTest.php b/tests/Feature/Billing/InvoiceNumberTest.php
new file mode 100644
index 0000000..7392337
--- /dev/null
+++ b/tests/Feature/Billing/InvoiceNumberTest.php
@@ -0,0 +1,98 @@
+<?php
+
+use App\Models\InvoiceSeries;
+use App\Services\Billing\InvoiceNumbers;
+use Illuminate\Support\Facades\DB;
+
+/**
+ * Gapless and ascending, which is a legal requirement rather than a preference.
+ *
+ * All of this exists because the obvious implementation — "the highest number
+ * that exists, plus one" — hands the same number to two sales that land
+ * together, and hands a used number back out after a row is removed. A number
+ * that has already been printed on a document somebody holds.
+ */
+it('hands out the seeded series in order', function () {
+    $s = InvoiceSeries::query()->where('kind', 'invoice')->firstOrFail();
+
+    $numbers = [];
+
+    foreach (range(1, 3) as $ignored) {
+        DB::transaction(function () use ($s, &$numbers) {
+            [$number] = app(InvoiceNumbers::class)->next($s, 2026);
+            $numbers[] = $number;
+        });
+    }
+
+    expect($numbers)->toBe(['RE-2026-0001', 'RE-2026-0002', 'RE-2026-0003']);
+});
+
+it('restarts a yearly series in a new year, and only then', function () {
+    // Checked against the stored year, not against "is it January": an
+    // installation that issued nothing for fourteen months must still restart,
+    // and one issuing at 23:59 on New Year's Eve must not restart the invoice
+    // that follows a minute later under the old year.
+    $s = InvoiceSeries::query()->where('kind', 'invoice')->firstOrFail();
+
+    DB::transaction(fn () => app(InvoiceNumbers::class)->next($s, 2026));
+    DB::transaction(fn () => app(InvoiceNumbers::class)->next($s, 2026));
+
+    [$next] = DB::transaction(fn () => app(InvoiceNumbers::class)->next($s->refresh(), 2027));
+
+    expect($next)->toBe('RE-2027-0001');
+
+    [$again] = DB::transaction(fn () => app(InvoiceNumbers::class)->next($s->refresh(), 2027));
+
+    expect($again)->toBe('RE-2027-0002');
+});
+
+it('never restarts a series that does not reset yearly', function () {
+    $s = InvoiceSeries::query()->where('kind', 'invoice')->firstOrFail();
+    $s->update(['yearly_reset' => false, 'next_number' => 41]);
+
+    [$number] = DB::transaction(fn () => app(InvoiceNumbers::class)->next($s->refresh(), 2026));
+    expect($number)->toBe('RE-0041');
+
+    [$across] = DB::transaction(fn () => app(InvoiceNumbers::class)->next($s->refresh(), 2027));
+    expect($across)->toBe('RE-0042');
+});
+
+it('keeps every Rechnungskreis on its own counter', function () {
+    // A credit note is not an invoice with a minus sign in front of it, and
+    // sharing a counter would put gaps in both.
+    $invoice = InvoiceSeries::query()->where('kind', 'invoice')->firstOrFail();
+    $credit = InvoiceSeries::query()->where('kind', 'credit_note')->firstOrFail();
+
+    [$a] = DB::transaction(fn () => app(InvoiceNumbers::class)->next($invoice, 2026));
+    [$b] = DB::transaction(fn () => app(InvoiceNumbers::class)->next($credit, 2026));
+    [$c] = DB::transaction(fn () => app(InvoiceNumbers::class)->next($invoice->refresh(), 2026));
+
+    expect($a)->toBe('RE-2026-0001')
+        ->and($b)->toBe('GS-2026-0001')
+        ->and($c)->toBe('RE-2026-0002');
+});
+
+/*
+ * There is deliberately no test for the "must be inside a transaction" guard.
+ *
+ * RefreshDatabase wraps every test in one, so DB::transactionLevel() is already
+ * above zero before the first line of any test here runs — the guard cannot
+ * fire, and a test written to prove it does would be green for a reason that
+ * has nothing to do with the guard. A test that passes for the wrong reason is
+ * worse than no test, so this note stands in its place.
+ */
+
+it('starts every seeded Rechnungskreis at one', function () {
+    // An installation with no series cannot issue an invoice at all, and
+    // finding that out at the moment of the first sale is the worst possible
+    // time.
+    //
+    // toEqual, not toBe: pluck returns them in whatever order the database
+    // hands them back, and the order of four seeded rows is not the assertion.
+    expect(InvoiceSeries::query()->pluck('prefix', 'kind')->all())->toEqual([
+        'invoice' => 'RE',
+        'credit_note' => 'GS',
+        'cancellation' => 'ST',
+        'deposit' => 'AZ',
+    ]);
+});