Dominoes
Tiles and suits

Dominoes (also known as bones, cards, men, pieces or tiles), are normally twice as long as they are wide, which makes it easier to re-stack pieces after use. A domino usually features a line in the middle to divide it visually into two squares, called ends. The value of either side is the number of spots or pips. In the most common variant (double-six), the values range from six pips down to none or blank. The sum of the two values, i.e. the total number of pips, may be referred to as the rank or weight of a tile; a tile may be described as "heavier" than a "lighter" one that has fewer (or no) pips.

Tiles are generally named after their two values. For instance, the following are descriptions of the tile 🁄 bearing the values two and five:

A tile that has the same pips-value on each end is called a double or doublet, and is typically referred to as double-zero 🀱, double-one 🀹, and so on. Conversely, a tile bearing different values is called a single.

Every tile which features a given number is a member of the suit of that number. A single tile is a member of two suits: for example, 🀴 belongs both to the suit of threes and the suit of blanks, or 0 suit.

In some versions the doubles can be treated as an additional suit of doubles. In these versions, the 🁡 belongs both to the suit of sixes and the suit of doubles. However, the dominant approach is that each double belongs to only one suit.

The most common domino sets commercially available are double six (with 28 tiles) and double nine (with 55 tiles). Larger sets exist and are popular for games involving several players or for players looking for long domino games.

The number of tiles in a double- n set obeys the following formula:

which is also the (n +1)th triangular number, as in the following table.

This formula can be simplified a little bit when n {\displaystyle n} is made equal to the total number of doubles in the domino set:

(n) (n + 1) 2 {\displaystyle {\frac {(n)(n+1)}{2}}}

The total number of pips in a double- n set is found by:

n (n + 1) (n + 2) 2 {\displaystyle {\frac {n(n+1)(n+2)}{2}}} i.e. the number of tiles multiplied by the maximum pip-count (n)

e.g. a 6-6 set has (7 × 8) / 2 = 56/2 = 28 tiles, the average number of pips per tile is 6 (range is from 0 to 12), giving a total pip count of 6 × 28 = 168
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