Double entry records each transaction in two places: what was received, and what was given up.
Buying a van for cash increases what the business owns and decreases its cash by the same amount. Two entries, one transaction.
Why it exists
Because the two sides must always agree, an error shows up as a mismatch rather than passing silently. A single-sided record has nothing to check against.
This is the entire reason the balance sheet balances. It is not a coincidence or a target; it is arithmetic.
Debits and credits
The two sides. The words are unhelpful — a debit is not "money out" and a credit is not "money in", despite what a bank statement implies. They simply mean left and right.
An owner does not need to work in them. Modern software records both sides from one action, and the terms surface only when an accountant explains an adjustment.
The vocabulary that follows from it
Journal entry — one recorded transaction with both sides. Manual journal entries are how corrections and year-end adjustments are made.
General ledger — the complete record of every entry, organised by account. Everything else is a summary of it.
Trial balance — a list of every account balance, checked to confirm the two sides still agree. Where they do not, something is wrong before any report is worth reading.
What it does not catch
A transaction recorded in the wrong account is still balanced, and the trial balance will not flag it. So the books can be internally consistent and still wrong — see an amount is in the wrong category.
Nor does it catch a transaction never recorded at all, which is why reconciliation against the bank matters separately.