During the nineteenth century, off-track gamblers would often play billiards while waiting to hear the results of a horse race.
Sometimes, if they agreed on the merits of a particular horse, the gamblers would pool their money in an effort to win a greater amount on one bet or to soften the blow of a loss.
The “pooled” money, both bet and won, was counted out on the playing surface of the billiard table, which the gamblers came to call their “pool table.”
That’s why billiards is played on a “pool table”.
