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

Rope.

You take a brick and a rope, hang the brick with the rope, now you have a straight rope line.
Carve something until it matches the rope.

Divide in units:
Wind a rope around a stick: draw a line on the rope. Undo the wind. Now you have a rope with marks distanced the same amount.

Make a flat something with high precision: pour molten metal in a bowl. Let it cool. Gravity will flatten the liquid, when it cools you have a flat surface. Take the flat piece of metal, paint it, rub it onto the desired object, remove material where the desired object is colored. Repeat until the desired object is evenly colored.

These are just some examples.