Physical balancing of dice requires use of isohedral polyhedra. The polyhedra traditionally used for the d20 and d30 are such polyhedra. Perfect physical balancing is still not possible, though, due to physical differences in numbers, small inaccuracies in molds, additional inaccuracies introduced during tumbling, and density variations due to defects like voids. In addition, it's possible to affect the roll of dice to a degree by carefully controlling the manner in which they're tossed. For these reasons, dice are more fair if they are numerically-balanced as well. For example, a void inside a die near a vertex (a point where three or more faces come together) will cause that vertex to preferentially face up when the die is tossed. The effect of such defects can be minimized by arranging the numbers such that the sum of the faces meeting at each vertex is the same.