I have an interest in the origins of words and phrases.

(IE The "Whole nine yards" seems to have come from either WW2 fighters were loaded with an ammo belt that, commonly, measured nine yards. If they returned with no ammo left, they gave them "the whole nine yards". OR A concrete truck holds nine yards of concrete.)

In football, we have fullbacks, halfbacks, quarterbacks, tailbacks, running backs ....

Why are they all called "backs"? 

My best guess is that they line up behind, in back of, the line of scrimmage?

Mod Edit: moved to “Other Sports”.