Nine-ball (sometimes written 9-ball) is a contemporary form of pool (pocket billiards), with historical beginnings rooted in the United States and traceable to the 1920s. The game may be played in social and recreational settings by any number of players (generally one-on-one) and subject to whatever rules are agreed upon beforehand, or in league and tournament settings in which the number of players and the rules are set by the sponsors. During much of its history, nine-ball has been known as a "money game" in both professional and recreational settings, but has since become established as a legitimate alternative to eight ball, straight pool and other major competition games.