Deuce in badminton is the score at 20-20, where the game no longer ends at 21 — you have to win by two points, up to a hard cap of 30. This explains exactly how deuce works, why 21-20 isn't a win, how a game can stretch to 24-22 or 29-29, and the golden-point rule at 29-29 where the very next rally wins it 30-29.
