BOOKS, commerce, accounts. Merchants, traders, and other persons, who are desirous of understanding their affairs, and of explaining them when necessary, keep, 1. a day book; 2. a journal; 3. a ledger; 4. a letter book; 5. an invoice book; 6. a cash book; 7. a bill book; 8. a bank book; and 9. a cheek book. The reader is referred to these several articles. Commercial books are kept by single or by double entry.