Nine ball is a my favorite game of pool. It hones the skill of reading the table, which strengthens the all-around game. Because the lowest numbered object balls must be contacted first by the cue ball, one is encouraged to plan three or even five shots ahead, so that the cue ball will travel to an ideal position for the next shot. The game also allows practice of combination shots, particularly when following a scratch.
A note on the thorough write-up above:
A legal break is one the contacts the number one ball first, then drives at least four object balls to the rail, or pockets any object ball.
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