In tennis, Deuce is when both players have 40 points in a game.
To win at Deuce, a player must score two points in a row. If a player scores one point at a deuce, they have the "Advantage." The score goes back to deuce if the other player scores the next point.
There is no limit to how many times a deuce can happen in a game. In 1975, Anthony Fawcett and Keith Glass played a record 37 deuces in one game! That added up to 80 total points before someone finally won.