Double-entry

Double-entry bookkeeping, in accounting, is a system of bookkeeping so named because every entry to an account requires a corresponding and opposite entry to a different account. For instance, recording earnings of $100 would require making two entries: a debit entry of $100 to an account named "Cash" and a credit entry of $100 to an account named "Revenue."

Deciding which account has to be debited and which account has to be credited is accomplished using the accounting equation which is expressed as, ${\displaystyle {\text{Assets}}={\text{Equity}}+{\text{Liabilities}}}$. This equation serves as an error detection tool; if at any point the sum of debits for all accounts does not equal the corresponding sum of credits for all accounts, an error has occurred.

Double-entry bookkeeping is no guarantee that errors have not been made; for example, the wrong ledger account may have been debited or credited, or the entries completely reversed.

The oldest record of a complete double-entry system is the Messari (Italian: Treasurer's) accounts of the Republic of Genoa in 1340. The Messari accounts contain debits and credits journalised in a form, and include balances carried forward from the preceding year, and therefore enjoy general recognition as a double-entry system. By the end of the 15th century, the bankers and merchants of Florence, Genoa, Venice and Lübeck used this system widely.

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