Fingers

Anyone who's read the Inheritance cycle may recall that dwarves have seven fingers on each hand. At one point, one of them explains to Eragon why their God caused this to be. Apparently, five was not enough and eight was too many.

In fact, seven is no better then five and eight would be a slight improvement,though I guess it would be rather unwieldy. You might also notice that it doesn't actually explain anything at all, but that's beside the point.

Twelve is easy to divide, and ten is easy to multiply. Which gives rise to all kinds of arguments over things like metric and imperial units, and is generally very annoying.

Twelve is easy to divide because it factors a lot of different ways, which is an intrinsic property of the number. But ten is only easy to multiply because we count in base ten. If we used base twelve, we'd have a number which was good for multiplying and dividing and there wouldn't be any arguments. So why don't we? Because fingers, that's why. The curious thing about the Inheritance reference is that if five is too few and eight too many, then there's no reason why it shouldn't be six. And six actually would be better than five.