diff --git a/README.md b/README.md
index bcb4d65..5f3537c 100644
--- a/README.md
+++ b/README.md
@@ -64,9 +64,9 @@ operating = "0.00"
 ```
 
 Events count from 1: the first `[[events]]` is event 1. An event is an
-entry (`lines`), a reversal (`reverses`), a close (`close`, below) or a
-bill (`bill`, below), each posting one entry; or one of the books'
-commands that post what is due (`run_depreciation`, `dispose_asset`,
+entry (`lines`), a reversal (`reverses`), a close (`close`, below), a
+bill or an invoice (`bill`, `invoice`, below), each posting one entry;
+or one of the books' commands that post what is due (`run_depreciation`, `dispose_asset`,
 `run_schedules`, below), which post as many entries as are due, none
 included; or a schedule's registration (`register_schedule`), which
 posts none: exactly one of them. A key that names an event (`reverses =
@@ -77,10 +77,11 @@ posts one entry, event N's is entry #N. A change that posts no entry (a
 rename, a policy) is a `[[changes]]` entry instead (below). A key the
 shape doesn't have is refused too, never ignored.
 
-An entry, a reversal, a close, a bill and a disposal are dated (`date`);
-the other commands date their own entries (a run its month's end) and
-take no date. An entry, a reversal, a close and a bill are described
-(`description`); a command describes its own entries and takes none.
+An entry, a reversal, a close, a bill, an invoice and a disposal are
+dated (`date`); the other commands date their own entries (a run its
+month's end) and take no date. An entry, a reversal, a close, a bill and
+an invoice are described (`description`); a command describes its own
+entries and takes none.
 
 The books are the engine's own (`ledger_store::Store`, in a directory of
 their own): each event is the command the books take it by, journaled as
@@ -238,7 +239,8 @@ the side gives the direction.
   original's tags.
 - `payee` on an event: the payee (by name, one of `setup.payees`) the
   entry paid. Only on an entry (`lines`): a reversal takes its
-  original's, a bill names its vendor, a close and a command have none.
+  original's, a bill names its vendor, an invoice its customer, a close
+  and a command have none.
 - `parties = [{ party, amount? }, { party, percent? }, ...]` on an event:
   the parties (by name, in `setup.parties`) the entry is with, each with
   its share: an `amount` (an exact decimal string), a `percent`, or
@@ -258,12 +260,24 @@ the side gives the direction.
   splits that close's net on its own instead (books from an engine that
   rounded each close by itself). A close must have something to close: it
   posts one entry like any other event.
-- `bill = { vendor, amount }` as an event: a bill from `vendor` (a name;
-  a payee's record if one has it) for `amount`, dated and due on the
-  event's date. It posts one entry: General Expenses (6999) debited and
-  Accounts Payable (2000) credited, each account created with that fixed
-  name when first needed, as the books create a control role's account
-  (B-32).
+- `bill = { vendor, amount }` or `bill = { vendor, lines = [{ amount,
+  description?, project? }, ...] }` as an event: a bill from `vendor` (a
+  name; a payee's record if one has it) for `amount` (one line, described
+  as the event) or its `lines` (exactly one of them; a line's description
+  is the event's when left out), dated and due on the event's date. It
+  posts one entry: General Expenses (6999) debited and Accounts Payable
+  (2000) credited, each account created with that fixed name when first
+  needed, as the books create a control role's account (B-32). A line's
+  `project` (a `setup.projects` name) puts its cost on that project: the
+  entry's expense side is one line per project named, the untagged lines'
+  as one, the payable line never tagged (B-37 item 8).
+- `invoice = { customer, amount }` or `invoice = { customer, lines = [...]
+  }` as an event: an invoice to `customer` (a name, its record added when
+  first named), as a bill is (the same `lines`), dated and due on the
+  event's date. It posts one entry: Accounts Receivable (1200) debited and
+  Revenue (4000) credited, each created with that fixed name when first
+  needed; a line's `project` puts its revenue on that project, the
+  receivable line never tagged (B-37 item 8).
 - `run_depreciation = { asset, period }` as an event: the asset's run for
   `period` (`"YYYY-MM"`), which posts what the schedule says is due
   through the month's end and the books don't carry, dated then: a
diff --git a/examples/projects-invoice-and-bill-lines-count-by-project.toml b/examples/projects-invoice-and-bill-lines-count-by-project.toml
new file mode 100644
index 0000000..2cc9b85
--- /dev/null
+++ b/examples/projects-invoice-and-bill-lines-count-by-project.toml
@@ -0,0 +1,182 @@
+# An invoice's or bill's lines count toward the projects they name.
+#
+# wanderbooks: rules.md B-37 item 8 (journal feature
+# `document_line_projects`); regression tests in ledger-core
+# `tests/spec_document_projects.rs`, ledger-store
+# `tests/document_projects.rs`, ledger-mcp `tests/spec_projects.rs`
+# (`invoice_and_bill_lines_count_toward_their_projects`).
+#
+# Expected values worked by hand. Invoice 1 bills 3,000.00 + 500.00 of
+# Zeta's design (3,500.00), 200.00 of Harbor's hosting and an untagged
+# 100.00 rush fee: 3,800.00, all Revenue (4000), the receivable untagged.
+# Bill 1 is 1,200.00 of Zeta's illustration and 50.00 of untagged
+# postage: 1,250.00 of General Expenses (6999). Bill 2, 300.00, names no
+# project. The customer's payment and the bill's payment move cash and
+# the receivable or payable only: no project figure. So Zeta nets
+# 3,500.00 - 1,200.00 = 2,300.00; Harbor 200.00 (both are client work,
+# so neither has an unrecovered cost: on spec, a hoped-for payment is no
+# receivable, B-37 item 4); the untagged lines 100.00 - 50.00 - 300.00
+# - 1,500.00 (rent) = -1,750.00; together 750.00, the P&L's net
+# (3,800.00 - 1,550.00 - 1,500.00). Dana was paid 1,250.00 from the bank,
+# whatever the bill's projects; the second bill is still owed.
+
+id = "projects-invoice-and-bill-lines-count-by-project"
+title = "An invoice's or bill's lines count toward the projects they name"
+rule = "Each line of an invoice or bill MAY name a project; its revenue (an invoice's) or cost (a bill's) MUST then count toward that project's figures exactly as a tagged line of a hand-built entry would, the receivable, the payable and their payments MUST count toward none, and every figure that is not a project's (the P&L, a payee's 1099 payments) MUST be what it would be with no project named."
+status = "CONVENTION"
+sources = [
+  "Job-order costing: a job cost sheet accumulates the revenue and the direct costs identified with one job, a supplier's invoice and the job's billing among them (management-accounting convention; wanderbooks docs/spec/rules.md B-37 items 3 and 8)",
+  "wanderbooks docs/spec/rules.md B-11 and B-14: a period's net income is its revenue less its expenses",
+  "Treas. Reg. §1.6041-1(a)(1)(iv) (docs/spec/sources/irs-1099-payment-method.md): a 1099 counts payments made, not how the books classify them",
+]
+tags = ["projects", "classes", "job-costing", "accrual", "invoices", "bills"]
+
+[setup]
+currency = "USD"
+fiscal_year_start = "01-01"
+basis = "accrual"
+rounding = "half-even"
+cash_accounts = ["1000"]
+
+[[setup.accounts]]
+code = "1000"
+name = "Bank"
+type = "asset"
+[[setup.accounts]]
+code = "1200"
+name = "Accounts Receivable"
+type = "asset"
+[[setup.accounts]]
+code = "2000"
+name = "Accounts Payable"
+type = "liability"
+[[setup.accounts]]
+code = "4000"
+name = "Revenue"
+type = "revenue"
+[[setup.accounts]]
+code = "6200"
+name = "Rent"
+type = "expense"
+[[setup.accounts]]
+code = "6999"
+name = "General Expenses"
+type = "expense"
+
+[[setup.projects]]
+name = "Zeta"
+kind = "client"
+[[setup.projects]]
+name = "Harbor"
+kind = "client"
+
+[[setup.payees]]
+name = "Dana Illustration"
+form = "1099-NEC"
+
+[[events]]
+date = "2026-03-02"
+description = "Invoice to Cedar"
+invoice = { customer = "Cedar Client", lines = [
+  { description = "Design", amount = "3000.00", project = "Zeta" },
+  { description = "Hosting", amount = "200.00", project = "Harbor" },
+  { description = "Rush fee", amount = "100.00" },
+  { description = "Revisions", amount = "500.00", project = "Zeta" },
+] }
+
+[[events]]
+date = "2026-03-09"
+description = "Dana's bill"
+bill = { vendor = "Dana Illustration", lines = [
+  { description = "Illustration", amount = "1200.00", project = "Zeta" },
+  { description = "Postage", amount = "50.00" },
+] }
+
+[[events]]
+date = "2026-03-20"
+description = "Cedar pays invoice 1"
+lines = [
+  { account = "1000", side = "debit", amount = "3800.00" },
+  { account = "1200", side = "credit", amount = "3800.00" },
+]
+
+[[events]]
+date = "2026-03-30"
+description = "Pay Dana's bill"
+payee = "Dana Illustration"
+lines = [
+  { account = "2000", side = "debit", amount = "1250.00" },
+  { account = "1000", side = "credit", amount = "1250.00" },
+]
+
+[[events]]
+date = "2026-03-31"
+description = "Studio rent"
+lines = [
+  { account = "6200", side = "debit", amount = "1500.00" },
+  { account = "1000", side = "credit", amount = "1500.00" },
+]
+
+[[events]]
+date = "2026-04-02"
+description = "Dana's second bill"
+bill = { vendor = "Dana Illustration", amount = "300.00" }
+
+[[expect]]
+query = "entry"
+entry = 1
+[expect.values]
+"1200.debit" = "3800.00"
+"4000.credit" = "3800.00"
+
+[[expect]]
+query = "entry"
+entry = 2
+[expect.values]
+"6999.debit" = "1250.00"
+"2000.credit" = "1250.00"
+
+[[expect]]
+query = "project"
+project = "Zeta"
+from = "2026-01-01"
+to = "2026-12-31"
+[expect.values]
+revenue = "3500.00"
+expenses = "1200.00"
+net = "2300.00"
+"to_date.revenue" = "3500.00"
+"to_date.expenses" = "1200.00"
+"to_date.net" = "2300.00"
+unrecovered_cost = "none"
+
+[[expect]]
+query = "project"
+project = "Harbor"
+from = "2026-01-01"
+to = "2026-12-31"
+[expect.values]
+revenue = "200.00"
+expenses = "0.00"
+net = "200.00"
+"to_date.revenue" = "200.00"
+"to_date.expenses" = "0.00"
+"to_date.net" = "200.00"
+unrecovered_cost = "none"
+
+[[expect]]
+query = "profit_and_loss"
+from = "2026-01-01"
+to = "2026-12-31"
+[expect.values]
+"4000" = "3800.00"
+"6200" = "1500.00"
+"6999" = "1550.00"
+net_income = "750.00"
+
+[[expect]]
+query = "vendor_1099_paid"
+from = "2026-01-01"
+to = "2026-12-31"
+[expect.values]
+"Dana Illustration" = "1250.00"