Straight rail, also called straight billiards, three-ball billiards, or the free game, is a discipline of carom billiards that is the most basic form of the game. The game is played on a pocketless unmarked billiard table, usually 10 by 5 feet (3.0 m × 1.5 m) in size, and three billiard balls, one, usually white, that serves as the cue ball for the first player, a second cue ball for the second player (differentiated by a spot or by being yellow), and an object ball, usually red. The object of the game is to score points by striking the player's assigned cue ball with a cue stick so it makes contact with both the opponent's cue ball and the object ball in the same stroke, known as a carom. Games are played to a predetermined number of points.