A handicap (or "odds") in chess is a way to enable a weaker player to have a chance of winning against a stronger one. There are many kinds of such handicaps, such as material odds, extra moves (i.e. the weaker player can play the first x number of moves at the beginning of the game), extra time on the chess clock, and special conditions (such as requiring the odds-giver to deliver checkmate with a specified chess piece or pawn). Various permutations of these, such as "Pawn and two moves", are also possible. Handicaps were quite popular in the 18th and 19th centuries, when chess was often played for money stakes, in order to induce weaker players to play for wagers. Today, except for time odds, handicaps are rarely seen. However, the very strong chess engine Rybka has recently played a series of odds matches against strong human players.