Luca Pacioli described three books to be used in bookkeeping in his 1494 treaties: The Day Book, the Journal, and the Ledger. All three of these "books" typically exist in modern financial systems.
- The Day Book is the original transaction repository, today in what is often referred to as the source system, or original book of record.
- The Journal contains debit/credit representations, and today is either part of the transaction system that interfaces to the accounting system, or is part of the accounting system.
- The Ledger is part of the accounting system, called the General Ledger. However, today's organizations have more than one ledger, because the source systems do posting processes to the customer, vendor, employee or other contract balances.