Pool (cue sports)
Rotation games

Rotation games require players to make legal contact with the lowest numbered ball on the table or a foul is called. The earliest rotation game, originally known as 61, started off as a variant of fifteen-ball pool during the mid-nineteenth century. The name "rotation" came from how the balls were placed around the table in its unracked offshoot Chicago. 61 has spawned many variations of its own such as American rotation, nine-ball, ten-ball, and Kelly pool. Of these, nine-ball is the most popular and the predominant professional game with ten-ball as the second-most prominent. There are many local and regional tours and tournaments that are contested with nine-ball. The World Pool-Billiard Association (WPA) publishes the world standardized rules. The European professional circuit has instituted rules changes to make it more difficult to achieve a legal break shot.

The largest nine-ball tournaments are the US Open Nine-ball Championship and the WPA World Nine-ball Championship for men and women. A hotly contested event is the annual Mosconi Cup, which pits invitational European and U.S. teams against each other in one-on-one and scotch doubles nine-ball matches over a period of several days. The Mosconi Cup games are played under the more stringent European rules, as of 2007.
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