A domino has two ends with each showing a number. Here to the right we count 18 (green) and 12 (red), what gives its name Eighteen-Twelve. The difference between these two ends is 18-12=6, what make this domino belonging to the group of difference-six.
Dominoes may only be played by putting ends with the same figure to each other. To continue here, we need a twelve or an eighteen. Dominoes with equal number on both ends we call doubles. They are always put across to an end with the same number. This allows to continue the game into four directions.
The direction of the game follows regional traditions (e. g. in Germany clockwise, while in Switzerland counter-clockwise, or how experienced pedagogues claim there: following the slap round the face …)
When it is a players turn he puts a domino to a fitting end. If this is not possible he sits out and takes a domino from the stack and it is the next players turn.
The figures of the ends showing open are added. If the result can be devided by the divisor according to the game (3, 5, 7, 9 oder 11) without leaving a rest, the player earns the value of the quotient. Example: the double-18 is played on divisor 11; a player achieves the sum of 110 by putting his domino. Divided by 11, the player scores with 10 points.
Using the simple formula of (n+1)×(n+2)÷2 one can calculate the number of dominoes of a game by replacing »n« by the highest number. An ordinary double-six domino game in Germany has got six as highest figure. That means (6+1)×(6+2)÷2=7×8÷2=7×4=28 dominoes. In Europe the double-9 is also known while in the USA double-12, double-15 and even double-18 are available.