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

Some of the first rulers and straight edges weren’t a solid piece. They were made of string or rope. We could easily observe that if you put tension on a string you’d get a straight line. If you trace that strings edge then you can transfer that line to the bing you’re cuting or shaping.

We actually still use this method with chalk lines in construction. A string coated in powdered chalk and stretched between two point. That string is then pluck so it snaps down onto the surface leaving a straight line of chalk.