Every transaction is written twice, once on each side. Money leaves here, money arrives there. The two sides must agree, and if they do not, something is wrong.
The consequence is the clever part. If you invent a number, the two sides start disagreeing. To hide one lie you need a second lie to balance it, and that one needs two more. Double-entry does not make lying impossible. It makes the lie grow, until it is too big to fit anywhere.
It was printed in Venice in 1494, in twenty-seven pages tucked inside a six-hundred-page mathematics encyclopedia by the friar Luca Pacioli. He did not invent it — Venetian merchants had been doing it for years. Printing it is what set it loose on the world.
Like an audit, it has one limit that never goes away: it checks a book against itself. Both sides of a lie can be written very neatly indeed. A perfectly balanced set of books can still balance around something that is not there.