The d120 is based on a polyhedron known as the disdyakis triacontahedron. These dice are perfectly numerically balanced, with the same sums for numbers around vertices of the same type. 120 is the maximum number of faces possible in an isohedron, excepting dipyramids and trapezohedra, which aren't workable solutions for physical dice with more than about 50 faces. That makes the d120 the ultimate "fair die". These dice are 2" in diameter and weigh 1/5 of a pound.