A tetractys, also called a tetraktys or decad, is an arrangement of 10 points in the form of a triangle with 1 point in the first row, two in the second, three in
the third, and four in the fourth. It is therefore a representation of the triangular
number 
.
It also describes the arrangement of pins in bowling.
The tetractys was a mystical symbol that was important to the Pythagoreans.
