Odds and Evens, sixes and sevens

We all know what odd and even numbers are but why are they so called? Why should a pair of things be considered even, but three things be seen as odd? It is a bit like left (weak) and right (good) but this can be partly explained by the fact that left-handed people (like me) are a small minority, and we can be awkward (this is because we must operate some things made for right-handed people). But with numbers the amount of odds and evens is the same. A mystery to be solved...all I know is my grand piano stays 'even' because it has an 'odd' number of legs!