Domino is a small tile with an arrangement of spots, similar to those on dice. One side is marked with a number, while the other is blank or identically patterned. Dominoes can be made from a variety of materials, including bone, ivory, and carved wood; modern sets are often made of plastic. Each domino has a unique set of open ends that can be used to create a chain of dominoes, called a layout.

Traditionally, the game of domino was played on a table with dominoes arranged in a layout, with one player playing each domino in turn. To play a domino, it must be placed on the layout so that its two exposed ends match up with the open ends of adjacent dominoes. In this way, the game is a chain reaction wherein each domino triggers another domino to fall, and so on. The first domino in the chain to fall is the target domino, and the rest of the players compete to have their own tiles match up with the target in order to claim a victory.

The most popular domino commercially available is the double-six set, with 28 pieces. Larger sets exist, however, and can be used to play very long chains. Typical large sets have either the double-nine or the double-12 inlay.

Each player draws a hand of dominoes, usually seven, and then plays a tile onto the layout in turn. The heaviest domino in the hand of the player who went before is called an opening double and is the one that starts the chain. Players may call for an opening double in the same way that they call out numbers when betting on a horse race, for example saying “double-six?” or “six-five?”

In scoring games, a player scores points by placing a domino end to end such that the touching open ends count the number of pips on the domino. For example, a domino placed vertically such as a 6-5 produces open ends of 4 and 5, resulting in a total of 12 points for the player. Players accumulate these points during the course of the game and the winner is whoever accrues the most points in a given round or after a specified number of rounds.

Despite its simple mechanics, domino has complex physics. When a domino is stood upright, it stores potential energy based on its position. As it falls, the stored energy is converted to kinetic energy as it moves along its path and causes other dominoes to fall. This transformation from potential to kinetic energy is why the smallest domino can cause such a dramatic effect when it is played correctly. A physicist at the University of Toronto has explained this principle by comparing the force exerted on an object to the force of gravity: an object in motion has more kinetic energy than an object that is at rest. This is the same reason that a ball dropped on the floor bounces up instead of falling straight down.