Dominoes spots from one to six are generally arranged as they are on six-sided dice, but because blank ends having no spots are used, seven faces are possible, allowing 28 unique pieces in a double-six set. However, this is a relatively small number especially when playing with more than four people, so many domino sets are "extended" by introducing ends with greater numbers of spots, which increases the number of unique combinations of ends and thus of pieces. Each progressively larger set increases the maximum number of pips on an end by three, so the common extended sets are double-nine, double-12,
Dominoes be played next to it must add up to a given number. For example, in a double-six set, the "sum" would be six, requiring a blank to be played next to a six, an ace (one) next to a five, a deuce (two) next to a four, etc. The stock of bones left behind, if any, is called the bone yard, and the bones therein are said to be sleeping. In draw games, players take part in the bone selection, typically drawing from the bone yard when they do not have a "match" in their hands. If a player inadvertently