U.S. Open pool championships and U.S. Open pocket billiards championships are generic terms that may refer to various professional pool tournaments, not all of them affiliated with each other. "U.S. Open Pocket Billiards Championship" as a proper noun most often refers to the straight pool (14.1 continuous pool) championship, the oldest of the events. Though "U.S. Open Pool Championship" as a stand-in for an official event name most commonly refers to the nine-ball event, it may, depending upon context, refer to any of six different annual tournaments, some of comparatively recent inception.