A domino is a small, thumb-sized rectangular block with one face blank and the other bearing an arrangement of dots, or pips, which look similar to those on a die. A complete set of dominoes contains 28 such blocks. Dominoes are used for a variety of games that can be played in many different ways. The most popular of these are blocking games, in which the object is to empty one’s hand, and scoring games, in which a player attempts to add up his or her points.
Traditionally, dominoes have been made of bone or ivory, and dark hardwoods such as ebony, with contrasting white or black pips inlaid or painted on. Today, plastic dominoes are more common. Nevertheless, in times past, sets were also made from other natural materials, such as marble, soapstone, and agate; metals, including brass and pewter; ceramic clay; and crystal or frosted glass.
The most common commercially available sets contain double six or double nine dominoes, although larger ones exist. Depending on the game being played, the number of dominoes required may be determined by the size of the board or by how long a player wishes to play a particular game. Dominoes are also available in an array of shapes, colors and designs; some are even printed with a musical theme.
A domino can be played with one, two or more players. In the most common games, the first player to empty his or her hand wins. The remaining players must then “block” the opponent’s play, keeping him or her from playing any more of his or her dominoes until the winner has scored a sufficient amount of points.
To begin a game, each player chooses the heaviest domino in his or her hand and places it on the table with its matching end touching another domino in such a way that their adjacent faces are either identical or form some specified total. The resulting chain of dominoes develops a snake-like shape as it grows and is added to. A domino placed to a double must be crossed-ways across the end of that domino; the matching ends are not allowed to touch squarely.
When a domino is knocked down, it triggers the fall of all the remaining pieces in its line. This is called the domino effect, and it is an example of a law of physics known as conservation of linear momentum. It is a key concept that helps explain how objects such as cars and baseballs can travel in straight lines while maintaining their relative speed.