A transfer that debits one account and crashes before crediting the other would make money vanish. ACID names the four guarantees that prevent this: all-or-nothing, valid data stays valid, transactions do not see each other's half-done work, and committed data survives crashes. Think in ACID terms whenever several writes must act as one unit, like booking a seat and charging a card. Remember that consistency also depends on your own code being correct, which is a favourite trap question.