When both players reach "40-40" for the first time in any given game, it is called "deuce." Then one player reaches "advantage." If the other player (the one without the advantage) wins the next point, it used to be called "advantage-all" (back when I played tennis - nowadays I think they just call it deuce again, without the "again" part). If the game continued for two more points (each one obviously won by each player) then the score went back to "deuce." The game ended when one player scored two points in a row after deuce was reached, whether or not it was the first time in that game. That's how I recall it, 30 years on.