The double-entry bookkeeping system was codified in the 15th century and refers to a set of rules for recording financial information in a financial accounting system in which every transaction or event changes at least two different accounts. In modern accounting this is done using debits and credits within the accounting equation: Equity = Assets - Liabilities. The accounting equation serves as a kind of error-detection system: if at any point the sum of debits does not equal the corresponding sum of credits, an error has occurred.