Cue sports playing nine-ball. Snooker balls are smaller than American-style pool balls with a diameter of 52.5 mm ( in), and come in sets of 22 (15 reds, 6 "", and a cue ball). English billiard balls are the same size as snooker balls and come in sets of three balls (two cue balls and a red, an object ball). Other games, such as bumper pool, have custom ball sets. Billiard balls have been made from many different materials since the start of the game, including clay, bakelite, celluloid, crystallite, ivory, plastic, steel and wood. The dominant material from 1627 until the
Rules of snooker play a shot that leaves the opponent unable to hit a ball "on" directly. This is most commonly called "snookering" one's opponent, or alternatively "laying a snooker" or putting the other player "in a snooker". Because players receive points for fouls by their opponents, repeatedly snookering one's opponent is a possible way of winning a frame when potting all the balls on the table would be insufficient to ensure a win or tie. This portion of the frame is known as the "snookers-required" stage. A free ball is a player-nominated substitute for the ball "on" when a player becomes snookered