Eli5 How do you create a ruler without a ruler?

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

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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.

Nothing is perfectly straight on an atomic level. You make things that are straight enough and work with what you have. Then you work on those things over time to make them slightly better for the job you’re trying to do.

You use a “[surface plate](https://en.m.wikipedia.org/wiki/Surface_plate)”. It’s a property of surfaces getting rubbed together consecutively that the only way you can get a common plane between 3 surfaces is to have one that is essentially perfectly flat.

So you start with nothing, then you rub two mostly flat rocks together, then you rub a third flat rock on the first one, then you rub the second and third ones together, and then you rub the first and second together again, repeating over and over, and eventually, you end up with a perfectly flat surface.

You can then use that to define the line along which you make your ruler.

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.

There are several ways of making flat things but let me explain one of the coolest and simplest ones. If you start off with two surfaces you can rub them together lapping them so they wear down the high spots of each other. Eventually the two surfaces will match each other perfectly. They will still not be flat, one will be slightly convex and the other slightly concave. But you can add a third surface and do the same. You lap it with the convex surface and it turns concave but then when you lap the two concave surfaces they both end up even closer to flat. Repeat this multiple times lapping the different surfaces to each other and you end up making them more and more flat.