Dominoes are small, rectangular blocks of wood or plastic marked with numbers or blank areas resembling those on dice. They are normally twice as long as they are wide. The values on either end of the domino are called pips. The value of a domino, or its rank or weight, is the sum of the values on both ends.

A player makes a play by placing one domino edge to edge against another, positioning it so that the two matching ends touch fully and form a chain that gradually increases in length. Each domino must have a number showing upon it or be a double, and a player cannot place a domino without touching the previous tile. The shape of the resulting snake-line chain varies according to the rules of the game being played.

When a domino is placed correctly, it may cause other dominoes to fall down in a line, creating a sequence of events called a Domino Effect or Domino Reaction. This happens because the energy stored in the first domino is transmitted to the next domino, causing it to push harder until the domino reaches its full potential energy or crashes down. Then the rest of the energy is converted to kinetic energy, causing it to push other dominoes in turn until all of them fall down.

If a Domino Effect is not wanted, the players can agree to change the rule. For example, some players may choose to count the total number of pips on all the tiles left in the losing player’s hand at the end of a hand or the game and add that number to the winner’s score. Another method is to count only the ends of a double, and not both of its edges.

Occasionally, the number of available dominoes is too limited to provide a large enough set for an entire game. Then an “extended” domino set is used, which adds more dominoes to the normal set by introducing new ends with additional pips. Most popular extended sets are double-nine, double- twelve, and double-18.

In a game with more than three or four players, seating arrangements at the table are decided by lot after the tiles are shuffled and then drawn. The player who draws the highest value tile (or a double, if that is what the rules call for) has first choice of seat. If a tie exists, it is broken by drawing additional dominoes from the stock. These extra tiles are then returned to the stock and reshuffled before the players draw their hands again.

Many domino games involve the use of a stock, which is a pile of unplayed dominoes that may be bought by any player for a particular point value. The number of pips on the dominoes in the stock is usually stated in the game’s rules. Some games also specify the amount of time a player may spend in purchasing dominoes from the stock before the game is over.