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Dominoes (or "dominos") generally refers to the individual or collective gaming pieces making up a domino set (sometimes called a deck or pack) or to the games played with these pieces. (In the area of mathematical tilings and polysquares the word domino often refers to any rectangle formed from joining two squares edge to edge.)
Standard domino sets consist of 28 pieces called bones, tiles, stones or dominoes. Each bone is a rectangular tile with a line dividing its face into two square ends. Each end is marked with a number of black spots (also called pips) or is blank. The spots are generally arranged as they are on six-sided dice, but because there are also blank ends having no spots there are normally seven possible faces.
Standard domino sets have ends ranging from zero spots to six spots (double six set), but specialized sets might range from zero to nine (double nine set), zero to twelve (double twelve set), zero to fifteen (double fifteen set), or zero to eighteen (double eighteen set).
The back side of a domino is generally plain. Dominoes have been made of bone, ivory, plastic, and wood, and occasionally are made of cardstock like that for playing cards. Dominoes are rather generic gaming devices--just as are playing cards. Many different games can be played with a set of dominoes.

Domino tiles and suits
Bones are generally named for the number of spots on the two ends of the bone. A bone with a 2 on one end and a 5 on the other end is called the 2-5, for example. Bones that have different numbers on the two ends are called singles, and bones that have the same number on both ends are called doublets or doubles. Bones that share a common number of spots on one end are said to be of the same suit. In a double-six set, for example, 1-0, 1-1, 1-2, 1-3, 1-4, 1-5, and 1-6 all belong to the suit of one. All singles belong to two suits. The 1-2, for example, belongs to the suit of one and the suit of two. All doubles belong to one suit only.

The ranks of domino pieces
The value of each end of a bone is determined by the number of spots on the end, with zero (blank) being the lowest and six being the highest. The rank of a bone is determined by the combined number of pips on the two ends. This rank is sometimes referred to as the bone's weight so that a higher ranking bone is called a heavier bone while a lower ranking bone is called lighter.

Playing a domino piece
4-6 played on 4-5The bones that are face up in play are called the layout, chain, or line. The layout will have one or more open ends that are available to be played upon. In most games, there are two open ends--one at each end of a line of bones. In some games there may be more, or there may be varying numbers depending upon the circumstances of play. In some games, the first doublet of each hand, often called the "sniff" or "spinner", forms the intersection of a cross in the layout. This usually means that there are four open ends once the doublet has been played.

When only a single bone has been played, the two open ends are generally the two ends of the bone. If Player A played a 4-5, for example, there is a 4 on one open end and a 5 on the other. The next player must usually play a bone with an end that matches one of the open ends. Player B, therefore, must play a bone with either a 4 or a 5, and the matching ends must touch. If Player B plays the 4-6, the new bone is placed with the two 4 ends touching so that the new open ends are 5 and 6. Doubles are placed crosswise and sprouted (played upon) crosswise. As the layout grows, the two ends of the layout generally form the two playable ends.

Common domino games
Most domino games are block games or draw games. In draw games, players draw from the boneyard when they have no matching bone. In block games, players pass and forfeit the turn when they have no matching bone. Otherwise, there is no difference. Both generally consist of several hands of dominoes played until one of the players accumulates an agreed upon number of points and wins the series. Points are generally earned only by the first player in each hand to go out (play his or her last bone, also called to domino) and win the hand. The primary object is thus to play all ones bones before an opponent does.

There are many existing rules for determining which player is the leader (or downer), the player to make the first play of the hand. In some rules, the lead is determined by lottery. The bones are shuffled face down on the table, and each player draws one bone. The player with the highest double, or heaviest bone, or other agreed upon prize is designated the leader. By this rule, the leader then reshuffles the bones before the final deal. By other rules, the final deal determines the leader. Playing the first bone of a hand is sometimes called setting the first bone, leading the first bone, downing the first bone, or posing the first bone, and the bone so set, led, downed, or posed is called the set, the lead, the down, or the pose. After the first hand, the winner of the previous hand is usually the leader for the next. By some rules, however, the lead rotates player to player across hands.

After the final shuffle the bones are dealt; each player in turn draws the number of bones required. The stock of bones left behind is called the boneyard, and the bones therein are said to be sleeping. If the leader was determined by lottery, the leader sets by placing any bone face up on the table. If the leader was not determined by lottery, the player with the highest double leads with that double, and if no player has a double, the hand is reshuffled and redealt.

The next player, and all players in turn, must play a bone with an end that matches one of the open ends of the layout. Play continues until one of the players goes out (and calls "out!" or "domino!") and wins the hand or until all the players are blocked. If all the players are blocked the player with the lightest hand wins.

In block games, players who cannot match on their turn must forfeit the turn by knocking (passing)--accomplished by rapping twice on the table or by saying, "go" or "pass". In draw games, players who cannot match must draw bones from the boneyard until obtaining a playable bone. According to most rules, the last two bones in the boneyard may not be drawn. If the boneyard is exhausted (only two bones left), the player knocks.

The winning player scores a point for each pip on each bone still held by each opponent. If no player went out, however, and the win was determined by the lightest hand, the winning player sometimes scores a point for each pip on each bone still held by each opponent, and sometimes only the excess held by opponents. A game is generally played to 100 points, the tally being kept with paper and pencil or on a cribbage board.

The origin of dominoes
Dominoes are descendants of dice. The two ends on each of the original Chinese dominoes represented one of the 21 combinations that can occur with the throw of two dice. Modern western dominoes, however, have blank ends on them as well and so the number of dominoes is generally 28. Dominoes were apparently unknown in Europe until the 18th century and may have been invented in their modern form in Italy. The dark spots on light faces apparently reminded people of masquerade masks with eyeholes (called dominoes) and thus gave the playing pieces their name. Chinese dominoes do not have blanks, but some whole tiles are duplicated..

Other uses of dominoes
Other than playing games of strategy, another common pastime using domino tiles is to stand them on edge in long lines, then topple the first tile, which falls on and topples the second, etc., resulting in all of the tiles falling. Arrangements of thousands of tiles have been made that have taken several minutes to fall. By analogy, similar phenomena of chains of small events each causing similar events leading to eventual catastrophe are called domino effects.

Source: Wikipedia - Article text licensed under the GFDL.