Except when specifically contradicted, all General Rules of Pocket Billiards.
Crossover is a rather unconventional 2-player game with very simple rules. It is challenging for any level of player, and cue ball control is critical.
One player arranges the rack as he/she chooses, but may not put both the 1 and the 15 in the interior three positions of the rack. The other player then chooses whether to begin with the 1 or with the 15 and breaks. The breaker may advance balls as on any other shot, but if no balls pass the center string (line between the side pockets) the breaker does not advance. The cue ball may contact any rails prior to touching the rack without penalty.
One player begins with the 1 ball as his/her target ball and advances toward the 15; the other begins with the 15 and advances toward the 1. In a turn, a player gets exactly one shot in which to advance as many balls as possible. Players advance by touching their target ball with the cue ball, or when their target ball is pocketed. Even within a shot, balls must be advanced in order. Advancement occurs at the time of contact, or at the moment the target ball is pocketed. Cue ball collisions with non-target balls do not effect advancement. All pocketed balls remain pocketed. A player automatically advances past any balls that are not on the table when they are reached. Shots are not called.
A scratch or any other foul results in a ball in hand anywhere on the table. Failing to make the cue ball contact an object ball on a shot is not a foul, but failing to touch any ball in two consecutive turns is a foul. Advancement in a turn is not negated by a scratch.
A player with a one-ball handicap does not need to touch the final ball (1 or 15), and so on.
In multi-player variations, all players try to advance past every ball on the table in increasing numerical order, with the 1 ball following the 15. With three players, one player begins with the 1 ball, another with the 6, and another with the 11. After choosing player order, the last player racks (no interior 1st balls), the breaking player chooses a starting ball (1, 6, or 11), then the 2nd player chooses a starting ball.
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