diff --git a/crates/ledger-core/src/ledger.rs b/crates/ledger-core/src/ledger.rs
new file mode 100644
index 0000000..ccb1e9a
--- /dev/null
+++ b/crates/ledger-core/src/ledger.rs
@@ -0,0 +1,225 @@
+//! The ledger: the command surface of the engine.
+//!
+//! A [`Ledger`] owns the chart of accounts and the append-only log of posted
+//! journal entries. Every mutation is a method on `Ledger` — create, add
+//! line, post, reverse — and every method validates before it applies. There
+//! is no other way to change the books, which is what makes the ledger an
+//! audit trail and what will make the future oracle harness trustworthy.
+
+use crate::{
+    AccountId, AccountType, ChartOfAccounts, Currency, EntryId, EntryStatus, JournalEntry,
+    JournalLine, LedgerError, Money, Side,
+};
+use chrono::NaiveDate;
+use rust_decimal::Decimal;
+use std::collections::HashMap;
+
+/// The books. Single-currency (see R-5); owns id allocation.
+#[derive(Debug)]
+pub struct Ledger {
+    base_currency: Currency,
+    chart: ChartOfAccounts,
+    entries: HashMap<EntryId, JournalEntry>,
+    /// Entry ids in posting order: the append-only log.
+    order: Vec<EntryId>,
+    next_account_id: AccountId,
+    next_entry_id: EntryId,
+}
+
+impl Ledger {
+    /// Open a new, empty ledger denominated in `base_currency`.
+    pub fn new(base_currency: Currency) -> Self {
+        Self {
+            base_currency,
+            chart: ChartOfAccounts::default(),
+            entries: HashMap::new(),
+            order: Vec::new(),
+            next_account_id: 1,
+            next_entry_id: 1,
+        }
+    }
+
+    /// The currency every account and line in this ledger uses.
+    pub fn base_currency(&self) -> Currency {
+        self.base_currency
+    }
+
+    /// Read-only view of the chart of accounts.
+    pub fn chart(&self) -> &ChartOfAccounts {
+        &self.chart
+    }
+
+    /// Add an account to the chart. Returns its id.
+    pub fn add_account(
+        &mut self,
+        code: &str,
+        name: &str,
+        account_type: AccountType,
+        parent: Option<AccountId>,
+        placeholder: bool,
+    ) -> Result<AccountId, LedgerError> {
+        let id = self.next_account_id;
+        self.next_account_id += 1;
+        self.chart.add(
+            id,
+            code,
+            name,
+            account_type,
+            parent,
+            placeholder,
+            self.base_currency,
+        )?;
+        Ok(id)
+    }
+
+    /// Start a draft journal entry. Returns its id.
+    pub fn create_entry(&mut self, date: NaiveDate, description: &str) -> EntryId {
+        let id = self.next_entry_id;
+        self.next_entry_id += 1;
+        self.entries.insert(
+            id,
+            JournalEntry::draft(id, date, description.to_string()),
+        );
+        id
+    }
+
+    fn draft_mut(&mut self, entry: EntryId) -> Result<&mut JournalEntry, LedgerError> {
+        let e = self
+            .entries
+            .get_mut(&entry)
+            .ok_or(LedgerError::UnknownEntry(entry))?;
+        if e.status() != EntryStatus::Draft {
+            return Err(LedgerError::PostedEntryImmutable);
+        }
+        Ok(e)
+    }
+
+    /// Add a line to a draft entry. The amount must be in the ledger's base
+    /// currency; drafts reject anything else rather than converting.
+    pub fn add_line(
+        &mut self,
+        entry: EntryId,
+        account: AccountId,
+        side: Side,
+        amount: Money,
+    ) -> Result<(), LedgerError> {
+        if amount.currency != self.base_currency {
+            return Err(LedgerError::CurrencyMismatch {
+                expected: self.base_currency.code().to_string(),
+                found: amount.currency.code().to_string(),
+            });
+        }
+        let e = self.draft_mut(entry)?;
+        e.push_line(JournalLine {
+            account,
+            side,
+            amount,
+        });
+        Ok(())
+    }
+
+    /// Validate and post a draft entry. One-way: a posted entry is immutable.
+    pub fn post_entry(&mut self, entry: EntryId) -> Result<(), LedgerError> {
+        let status = self
+            .entries
+            .get(&entry)
+            .ok_or(LedgerError::UnknownEntry(entry))?
+            .status();
+        if status == EntryStatus::Posted {
+            return Err(LedgerError::EntryAlreadyPosted(entry));
+        }
+        // Clone so validation borrows end before the mutable mark.
+        let snapshot = self
+            .entries
+            .get(&entry)
+            .ok_or(LedgerError::UnknownEntry(entry))?
+            .clone();
+        snapshot.validate(&self.chart)?;
+        self.entries
+            .get_mut(&entry)
+            .ok_or(LedgerError::UnknownEntry(entry))?
+            .mark_posted();
+        self.order.push(entry);
+        Ok(())
+    }
+
+    /// Look up an entry by id (draft or posted).
+    pub fn get_entry(&self, entry: EntryId) -> Option<&JournalEntry> {
+        self.entries.get(&entry)
+    }
+
+    /// Post a reversing entry: same lines with sides flipped, linked back to
+    /// the original via `reversal_of`. The books never forget; they correct.
+    pub fn reverse_entry(
+        &mut self,
+        entry: EntryId,
+        date: NaiveDate,
+        description: &str,
+    ) -> Result<EntryId, LedgerError> {
+        let original = self
+            .entries
+            .get(&entry)
+            .ok_or(LedgerError::UnknownEntry(entry))?
+            .clone();
+        if original.status() != EntryStatus::Posted {
+            return Err(LedgerError::EntryNotPosted(entry));
+        }
+        let id = self.create_entry(date, description);
+        for line in original.lines() {
+            let flipped = match line.side {
+                Side::Debit => Side::Credit,
+                Side::Credit => Side::Debit,
+            };
+            self.add_line(id, line.account, flipped, line.amount)?;
+        }
+        self.entries
+            .get_mut(&id)
+            .ok_or(LedgerError::UnknownEntry(id))?
+            .mark_reversal_of(entry);
+        self.post_entry(id)?;
+        Ok(id)
+    }
+
+    /// Posted entries in posting order: the append-only log.
+    pub fn posted_entries(&self) -> impl Iterator<Item = &JournalEntry> + '_ {
+        self.order.iter().filter_map(|id| self.entries.get(id))
+    }
+
+    /// Signed net movement of an account across all posted entries:
+    /// debits minus credits. Positive means net debit movement.
+    pub fn account_movement(&self, account: AccountId) -> Decimal {
+        let mut total = Decimal::ZERO;
+        for entry in self.posted_entries() {
+            for line in entry.lines() {
+                if line.account == account {
+                    match line.side {
+                        Side::Debit => total += line.amount.amount,
+                        Side::Credit => total -= line.amount.amount,
+                    }
+                }
+            }
+        }
+        total
+    }
+
+    /// Balance expressed in the account's normal side: positive means a
+    /// normal balance, negative means a contra balance.
+    pub fn account_balance(&self, account: AccountId) -> Result<Decimal, LedgerError> {
+        let account = self
+            .chart
+            .get(account)
+            .ok_or(LedgerError::UnknownAccount(account))?;
+        let movement = self.account_movement(account.id);
+        let balance = match account.account_type.normal_side() {
+            Side::Debit => movement,
+            Side::Credit => -movement,
+        };
+        // Never leak negative zero into reports: it is a representation
+        // artifact of negating an exact zero, and money must never print "-0".
+        Ok(if balance.is_zero() {
+            Decimal::ZERO
+        } else {
+            balance
+        })
+    }
+}