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,
Schläfli double six formed in this way are all in turn intersected by another line, "a". The twelve lines "a" and "b" form a double six: each line "a" has an intersection point with five of the other lines, the lines "b" for which "i" ≠ "j", and vice versa. An alternative construction, shown in the illustration, is to place twelve lines through the six face centers of a cube, each in the plane of its face and all making the same angles with respect to the cube's edges. A generic cubic surface contains 27 lines, among which can be found 36 Schläfli