Double-Entry Accounting

Double-entry accounting is the method of bookkeeping in which every transaction is recorded in at least two accounts, with total debits equal to total credits. Buying a $1,000 laptop for cash, for example, increases equipment by $1,000 and decreases cash by $1,000. Because both sides are always recorded, the books stay in balance and many errors show up automatically.

It is the basis of almost all modern accounting, from small firms to banks and governments, and the frameworks behind company reporting, IFRS Accounting Standards and US GAAP, assume it. It is often contrasted with single-entry bookkeeping, a simple list of money in and out. An informal name for the same method is double accounting. Its records are kept in journal entries and the general ledger.

Every deal has two sides: something comes in and something goes out, or something is gained and something is owed. If you borrow a friend's bicycle, you now have a bicycle and you also owe a bicycle. Double-entry accounting writes down both halves every time.

Because both halves are always written down, the two sides of the record must match at the end of the day, like the two pans of a balanced kitchen scale. If they don't match, the bookkeeper knows something has been missed and can go looking for it before the numbers reach anyone else.

Double-entry rests on one identity:

• $\text{Assets}$ are resources the business controls, such as cash, receivables and equipment; see asset.
• $\text{Liabilities}$ are amounts it owes to others, such as loans and supplier bills; see liabilities.
• $\text{Equity}$ is the owners' residual claim: capital put in plus retained profits.

Every transaction changes at least two items in a way that keeps the equation true. Worked example: a new company starts with $10,000 of owner's capital (assets $10,000 = liabilities $0 + equity $10,000). It borrows $5,000 from a bank (assets $15,000 = liabilities $5,000 + equity $10,000). It buys $3,000 of equipment with cash; cash falls by $3,000 and equipment rises by $3,000, so assets stay $15,000. It earns $2,000 of fees in cash; assets rise to $17,000 and equity rises to $12,000 through net income. The equation balances at every step, and the result is the balance sheet.

A debit is an entry on the left side of an account and a credit is an entry on the right. Whether each increases or decreases a balance depends on the account type:

Revenue and expenses are temporary parts of equity: revenue raises it, expenses lower it. This explains a point that often confuses people: when a bank "credits" a customer's account, it is increasing a liability in its own books, because the bank owes that money to the customer. The same deposit is a debit (an asset) in the customer's own books.

Since debits equal credits for each entry, total debits must equal total credits across the whole ledger. That check is the trial balance.

Double-entry bookkeeping developed among merchants and banks in the trading cities of northern Italy in the late 13th and 14th centuries, an era of trade in coins such as the Florentine florin and the Venetian ducat. The first printed description came in 1494, when the Franciscan friar and mathematician Luca Pacioli published Summa de arithmetica, geometria, proportioni et proportionalita in Venice. Its bookkeeping section described what Pacioli called the method of Venice: a memorandum book, a journal and a ledger, with each entry posted twice. Pacioli did not invent the method, but his text spread it across Europe.

Over the following centuries the method became standard for companies, and from the 19th century company law and auditing grew up around it. Some countries now write it into statute: section 128 of India's Companies Act 2013 requires company books to be kept on an accrual basis and according to the double-entry system.

Accounting software and ERP systems enforce double entry automatically: an entry that does not balance cannot be posted. The same principle underpins the ledgers of banks and payment firms. In a core banking system, a transfer debits one customer's account and credits another's, so money is never created or lost inside the bank. Fintech engineers apply double-entry design to wallet and payments ledgers for the same reason, and pair it with idempotency so a retried request does not post twice.

Double entry does not guarantee correct books. It cannot catch a transaction that was never recorded, an entry posted to the wrong account of the same type, or a deliberate fraud recorded with balanced figures. That is why it is combined with reconciliation, review and audit.

• IFRS Foundation: Conceptual Framework for Financial Reporting