Actually it can be both. Shapes work best in a casino setting. Now picture a pyramid shaped object, throw it, and of course it will land on the larger flat side almost all of the time. Now start to expand the sides of the object till it looks proportionet to the human eye. The shape you are striving for will still have a larger bottom than the other sides. It will favor the larger surface more than the other sides. The number you want to throw will be the top of the larger flat side. The other sides will still come come up, but the die will favor the larger flat side, thus percentage dice. I can draw this a lot easier that I can explain it.