I was reading an old sewing manual from 1978 and found out the average zipper is 7 inches long. That seems so specific.

Oh wow, that's a weird fact. I remember reading somewhere that a lot of zipper lengths got standardized for clothing patterns back then, so factories just made a ton of that one size. Makes sense they'd pick a middle length that works for dresses and pants. Kind of funny to think about a whole industry just agreeing on 7 inches.