Source documents are the original records that document a business transaction. Their main purpose is to provide evidence of what happened and to support the amounts recorded in the books. They capture essential details such as date, parties involved, amounts, and the nature of the transaction, and they feed into the journal entries that ultimately flow into the ledger. This creates an audit trail that can be traced from financial statements back to the underlying documents, helping ensure accuracy, prevent errors, and detect potential fraud. They are not optional; without them entries would lack verifiable support. They do not replace journal entries—journal entries are the formal recording in the books, while source documents supply the information used to make those entries. They are not used only for external reporting; they underpin day-to-day recording and internal controls as well.