normal balance
Every account has a 'home side' — the side it usually leans toward, where its balance is expected to sit. This is its normal balance. For Cash, that home side is the debit (left) side, because a business normally has cash rather than negative cash. Knowing each account's normal balance is like knowing which way is up: it tells you at a glance whether a number looks healthy or strange.
The normal balance always matches the side that increases that type of account. Assets and expenses increase with debits, so their normal balance is a debit balance. Liabilities, equity, and revenue increase with credits, so their normal balance is a credit balance. After all the debits and credits in an account are netted, the leftover should normally sit on this expected side. A Cash account ending with a credit balance, for instance, would mean the books show more cash going out than came in — usually a sign of an overdraft or, more often, an error.
Normal balances are the practical glue between the abstract debit/credit rules and real bookkeeping. They let an accountant sanity-check a ledger instantly: an Accounts Payable (a liability) with a debit balance, or a Sales Revenue with a debit balance, is a red flag worth investigating. The concept also drives how financial statements are built, since each line item is expected to carry the balance that its account type produces.
Accounts Payable is a liability, so its normal balance is a credit. If a company's Accounts Payable shows a debit balance after posting, that is unusual — perhaps a supplier was overpaid, or a payment was entered against the wrong account.
A balance on the 'wrong' side is not impossible, but it is a flag to investigate.
Contra accounts (like Accumulated Depreciation) deliberately carry the opposite of their category's usual normal balance, so 'normal' is per account, not blindly per category.