diff --git a/crates/ledger-core/src/account.rs b/crates/ledger-core/src/account.rs
new file mode 100644
index 0000000..7bb747f
--- /dev/null
+++ b/crates/ledger-core/src/account.rs
@@ -0,0 +1,148 @@
+//! Chart of accounts: the five account types and the account registry.
+//!
+//! Every account has exactly one of five types. The type determines the
+//! account's *normal balance side*: assets and expenses are debit-normal,
+//! liabilities, equity, and revenue are credit-normal. Reports are built by
+//! grouping on type, so getting this right is the whole game.
+
+use crate::{Currency, LedgerError};
+use serde::{Deserialize, Serialize};
+use std::collections::HashMap;
+
+/// Opaque account handle. Created by [`ChartOfAccounts`]; never forged.
+pub type AccountId = u64;
+
+/// The five kinds of accounts. This list is closed: all of double-entry
+/// bookkeeping is expressed with these.
+#[derive(Clone, Copy, PartialEq, Eq, Debug, Serialize, Deserialize)]
+pub enum AccountType {
+    /// What the business owns or is owed: cash, receivables, equipment.
+    Asset,
+    /// What the business owes: payables, loans.
+    Liability,
+    /// The owners' claim: capital, retained earnings.
+    Equity,
+    /// Increases to equity from operations: sales, fees.
+    Revenue,
+    /// Decreases to equity from operations: rent, wages, supplies.
+    Expense,
+}
+
+/// Debit or credit: the two sides of every journal line.
+#[derive(Clone, Copy, PartialEq, Eq, Debug, Serialize, Deserialize)]
+pub enum Side {
+    /// Left side. Increases assets and expenses.
+    Debit,
+    /// Right side. Increases liabilities, equity, and revenue.
+    Credit,
+}
+
+impl AccountType {
+    /// The side that increases an account of this type — its normal balance.
+    pub fn normal_side(&self) -> Side {
+        match self {
+            AccountType::Asset | AccountType::Expense => Side::Debit,
+            AccountType::Liability | AccountType::Equity | AccountType::Revenue => Side::Credit,
+        }
+    }
+
+    /// True for the three types that appear on the balance sheet.
+    pub fn is_balance_sheet(&self) -> bool {
+        matches!(
+            self,
+            AccountType::Asset | AccountType::Liability | AccountType::Equity
+        )
+    }
+}
+
+/// A single account in the chart of accounts.
+#[derive(Clone, Debug, Serialize, Deserialize)]
+pub struct Account {
+    /// Opaque handle.
+    pub id: AccountId,
+    /// Human-meaningful code, e.g. `"1000"`. Unique within the chart.
+    pub code: String,
+    /// Display name, e.g. `"Cash"`.
+    pub name: String,
+    /// One of the five types. Determines reporting and normal side.
+    pub account_type: AccountType,
+    /// Hierarchy link, e.g. `"1010"` under `"1000"`. `None` for top-level.
+    pub parent: Option<AccountId>,
+    /// Header accounts organize the chart and take no postings.
+    pub placeholder: bool,
+    /// Day one: always the ledger's base currency (see R-5).
+    pub currency: Currency,
+}
+
+/// The registry of accounts. Owns id allocation for accounts it holds.
+#[derive(Clone, Debug, Default)]
+pub struct ChartOfAccounts {
+    accounts: HashMap<AccountId, Account>,
+    by_code: HashMap<String, AccountId>,
+}
+
+impl ChartOfAccounts {
+    /// Register an account. Codes must be unique and non-empty; a parent, if
+    /// given, must already exist.
+    pub fn add(
+        &mut self,
+        id: AccountId,
+        code: &str,
+        name: &str,
+        account_type: AccountType,
+        parent: Option<AccountId>,
+        placeholder: bool,
+        currency: Currency,
+    ) -> Result<(), LedgerError> {
+        if code.is_empty() {
+            return Err(LedgerError::EmptyAccountCode);
+        }
+        if self.by_code.contains_key(code) {
+            return Err(LedgerError::DuplicateAccountCode(code.to_string()));
+        }
+        if let Some(p) = parent {
+            if !self.accounts.contains_key(&p) {
+                return Err(LedgerError::UnknownParentAccount(p));
+            }
+        }
+        self.by_code.insert(code.to_string(), id);
+        self.accounts.insert(
+            id,
+            Account {
+                id,
+                code: code.to_string(),
+                name: name.to_string(),
+                account_type,
+                parent,
+                placeholder,
+                currency,
+            },
+        );
+        Ok(())
+    }
+
+    /// Look up an account by id.
+    pub fn get(&self, id: AccountId) -> Option<&Account> {
+        self.accounts.get(&id)
+    }
+
+    /// Look up an account by code.
+    pub fn find_by_code(&self, code: &str) -> Option<&Account> {
+        self.by_code.get(code).and_then(|id| self.accounts.get(id))
+    }
+
+    /// Iterate over all accounts (order unspecified; sort by code for display).
+    pub fn all(&self) -> impl Iterator<Item = &Account> {
+        self.accounts.values()
+    }
+
+    /// Number of accounts in the chart.
+    pub fn len(&self) -> usize {
+        self.accounts.len()
+    }
+
+    /// True when the chart holds no accounts.
+    pub fn is_empty(&self) -> bool {
+        self.accounts.is_empty()
+    }
+}