The Schläfli double six is a configuration consisting
of two sextuples of skew lines on the general cubic
surface such that each line of one is skew to one
line in the other set. In all, there are 30 points, with
two lines through each point, and 12 lines with five points on each line. Two lines
can be placed in the plane of each of the faces of a cube. The double six was discovered
by Schläfli.