Eli5 How do you create a ruler without a ruler?

Question

How do you create a thing which is straight, when you don’t have a thing which is straight to create it with?

In: 4

Answer 1

I’m sure that there are many ways. One simple method would be to use gravity. Suspend a weight from a string and let it hang down. Gravity will stretch that string down in a straight line.

EDIT: Hell, you don’t even need gravity. If you grab both ends of the string and pull, the string will straighten out.