An audit trail is the documented flow of a transaction. It is used to investigate how a source document was translated into an account entry, and from there was inserted into the financial statements of an entity. The audit trail can be used in reverse, to track backwards from a financial statement line item to the originating source document. A well-run accounting system should have a clear audit trail for all transactions. An audit trail is used by both external and internal auditors to trace transactions through an accounting system, as well as by the accounting staff to track down errors and the causes of variances in the financial statements.