The double-entry system is the foundation of modern accounting. Every transaction is recorded in at least two accounts — one debited and one credited — and the amounts must always be equal.
For example, if you buy equipment for $10,000 in cash, you debit "Equipment" (+$10,000 asset) and credit "Cash" (-$10,000 asset). The books stay balanced because every debit has a matching credit.
This system, first described by Luca Pacioli in 1494 ensures accuracy, catches errors, and provides a complete picture of where money comes from and where it goes. Virtually every business in the world uses it.