A rack (sometimes called a triangle) is a piece of equipment that is used to place billiard balls in their starting positions at the beginning of a pocket billiards game. Rack may also be used as a verb to describe the act of setting billiard balls in their starting positions (e.g. "to rack the balls"), or as a noun to describe a set of balls that are in their starting positions (e.g. "a rack of balls", more often called a pack or a pyramid in British English).

Traditional racks are in the form of triangular frames, usually made from wood, plastic or metal. A modern variation, called a template rack, is made from a thin material (usually 0.14 mm or less) that contains precision cut-outs to hold the balls in place. Purported benefits of template racks include a more consistent racking, and their popularity has warranted specific inclusion in profession rules. Unlike traditional racks, template racks are left on the table during the break shot and removed at the players' earliest convenience. For this reason, template racks are almost never used for games where it is common to slow-break (i.e. not create a large spread of balls) since it is significantly more likely that the rack will interfere with slow-rolling balls.

The most common shape of a rack is that of an equilateral triangle. Triangular racks are used for eight-ball, straight pool, one-pocket, bank pool, snooker and many other games. Diamond-shaped frames are sometimes used for the game of nine-ball (although a triangular rack can also be used) and template racks come in a variety of shapes.

In eight-ball, 15 object balls are used. Under the world standardized rules, it is prescribed that:

In eight-ball, the rack is in the shape of an equilateral triangle since the number 15, also the count of the balls required to make the rack, is a triangular number (5 + 4 + 3 + 2 + 1 = 15)

In nine-ball, the basic principles are the same as detailed in the eight-ball section above, but only balls 1 through 9 are used; the 1 ball is always placed at the rack's apex (because in nine-ball every legal shot, including the break, must strike the lowest numbered ball first) over the table's foot spot, and the 9 ball is placed in the center of the rack.

Some players (most often amateurs) place the balls in numeric order but for the 9 ball; from the top of the triangle down and from left to right, i.e., the 1 on the foot spot, followed by the 2 then 3 in the second row, and so on. However, all balls other than the 1 and 9 may be randomly placed.

In nine-ball games where a handicap is given by one player being spotted a ball, some tournament venues enforce a rule that the spotted ball must be racked as one of the two balls in the row directly behind the 1 ball.