This question may have already been posed here, but I couldn’t find anything using the search. Google wasn’t much help, either.
Anyways, my question is about straight edges. Today we have rulers, levels, and all sorts of instruments to draw and measure straight lines with. Well, how was the very first one made? How did they know it was straight? What did they measure it against?
You don’t find many straight lines in nature, do you? I suppose one way to measure would be by stretching out a piece of string or hair, or water in a bucket.
Another way is to take three plates of something rigid but machineable, make them as flat as you can by eye, coat them with blacking or something, and rub two of them together. The blacking will rub off the high spots, so grind down the visible high spots and repeat with another pairing. If you used only two plates you would end up with one convex and one concave, but with three, alternating between AB, BC and CA, they will all end up dead flat. Now you have something to check your straight edges against.
A piece of gut, perhaps the small intestine of an animal, soaked in water, then twisted and stretched is the first type of cord made. A line of that cord, stretched between any two rigid objects, or by a weight suspended gives you straight, or vertical.
When you have your two rocks rubbed mutually flat, you stretch a hide across one, press the other on top, let it dry, and you have a flat hide. Then you can rub mud on your cord, stretch it out, and twang it against the flat rock, and you have drawn a straight line between arbitrary points. The same cord can give you arcs as well, although accuracy depends on the sharpness of your compass points, and how easily your cord slides around your fulcrum.
You have now the entire set of tools needed to do classic Greek geometry. Shouldn’t take you more than ten or twelve thousand years.
If all you want is “something to check your straight edges against”, you don’t need any tools at all. Just put one end of the straightedge close to your eye, and the other end far away. Holding it in this position will magnify any imperfections and make them very easy to find. Try it!
What makes you think your folds are really straight? It’s definitely not guaranteed.
(for kicks, I just tried folding two sheets of paper longways and comparing the folded edges. they definitely weren’t straight – there was about 1mm of cumulative error between the two of them over the course of 11 inches)
This method is still commonly used in the machining world. But two surfaces are plenty, if you slide them around enough to avoid the concave/convex issue. And they use a compound called “blueing” not “blacking”…TRM
snapping a chalk line creates a straight line and pulling a string tight in a bow saw configuration creates a straight edge. You could turn wood in a homemade lathe to form rods of fairly precise diameters using a gouge mounted in a fixed caliper.
A roughly constructed wood plane would cut a pretty good straight edge. It may be a function of creating a better wood plane which would then cut a better straight edge.