Sicherman Dice

http://commons.wikimedia.org/wiki/Category:Die_faces

What’s unusual about these dice is obvious. But what’s normal about them?

When thrown together, they produce the same probability distribution as a pair of ordinary dice. There are six ways to throw a 7, five ways to throw an 8, etc. This the only possible alternate arrangement in which all the face values are positive.

They were discovered/invented by George Sicherman of Buffalo, N.Y.