The domino is a small, flat block used as gaming object. It is sometimes referred to as bones, cards, men or pieces, and it may be colored or plain. The identifying feature of a domino is an arrangement of dots (or pips) on one side, with the other face either blank or identically patterned. A domino is normally twice as long as it is wide. It is also recognizable by its shape, which is characterized by a ridge that divides it visually into two squares of equal size. The value of each square is indicated by its number of spots, with a blank domino having no value at all.

The most common type of domino set has a maximum of nine pips on each end. A larger number of pips would allow for greater flexibility in game play, but it is uncommon to see sets with more than twelve pips. The pips may be painted, inlaid or otherwise marked on the surface of the domino. Traditionally, the surface of a domino is made from a material that will be durable for a long period of time. Some examples include bone, silver lip ocean pearl oyster shell (mother of pearl), ivory or a dark hardwood such as ebony, with contrasting black or white pips. A variety of polymer materials is also now available, for instance, bakelite and celluloid.

When a player places a domino, the open end of the tile must match the pips of the adjacent tiles to form a line that is called the layout, string or line of play. In many games the line of play continues until a player has placed all of his or her dominoes. When this happens, the count is completed and the score calculated.

As each domino is played, much of its potential energy converts into kinetic energy, the energy of motion (see Converting Energy). This energy is transmitted from the first domino to the next, providing the push needed to knock it over. The energy is then transmitted to still more dominoes, and so on, in a chain reaction that continues until all the dominoes have fallen.

There are numerous domino games, but most fall into one of four categories: bidding, blocking and scoring. Each category involves different strategies and tactics.

When a player draws a hand of dominoes, the heaviest domino makes the first play. This is usually determined by the rules of the specific game. In case of a tie, the player with the highest double or higher wins. Some games have rules that break ties by drawing additional dominoes from the stock.