A domino is a flat thumbsized rectangular block, the face of which is divided into two parts, each either blank or bearing from one to six pips or dots, resembling those on dice. 28 such dominoes form a complete set. A domino game is any of a number of games played with these pieces, typically by matching the ends of pieces and laying them down in lines or angular patterns. The term domino also refers to the set of rules governing such games, which may differ from place to place.
There are many different kinds of domino, but most fit into one of four categories: bidding games, blocking games, scoring games, and round games. Most of these games have very similar, but not identical, rules. The exact rules of a given domino game depend on how the tiles are laid out and which pieces are left undisturbed when another player plays.
To make a domino, players take turns placing a tile edge-to-edge against a matching one on the table. This arrangement is called the layout, string, or line of play. Each tile must have its open end facing toward the next piece played, or else it is considered a misplay. The first player to make a play is referred to as the setter, downer, or lead. The other players then determine in turn who will make the next play.
Depending on the rules of a particular game, a domino can be played in either an up-and-down or a side-to-side direction. In the up-and-down orientation, a player must touch all of the exposed sides of a domino before playing it. The shortest distance between two adjacent sides of the same domino is known as the “length” of the domino. A domino is said to be “played to length” if it touches the end of a previous tile in its line, or if its open end is touching another tile, a double.
The word domino is derived from the Italian domanda, meaning “fate.” A common belief is that the first domino to fall starts a chain reaction. In fact, the chain reaction is a result of the energy in the initial domino being converted from potential to kinetic energy. This energy is transferred from the initial domino to the subsequent ones, just as a nerve impulse travels along an axon and causes other neurons to fire.
A typical domino is twice as long as it is wide, which makes it easier to re-stack the pieces after use. Traditionally, dominoes are made from a material such as bone, silver lip ocean pearl oyster shell (mother of pearl), ivory, or dark hardwood such as ebony. More recently, polymer materials have been used to make dominoes. However, some sets are still manufactured from natural materials such as stone (e.g., marble or granite); soapstone; other woods such as ash or oak; metals (e.g., brass or pewter); ceramic clay; and even frosted glass.