system
Given the following post, respond as if you're commenting on Reddit:
human
How did engineers/machinists create the first precise straight edge?

If I had to create a good straight edge from scratch, my first thought would be to pour molten tin on top of molten glass for a flat surface, then fold the cooled tin sheet, as it seems from experience that folding creates a straight edge, though I've never heard how precise it is.

But I assume the above process for making a flat surface may not have been known in antiquity, so I'm wondering how and if they did it.
gpt
If you rub two relatively flat surfaces together you end up with a flat surface. Imagine 2 pieces of sandstone rubbed together until "all" points of the surfaces are touching. "All" is in quotes because obviously there are always surface imperfections which will keep the parts separated. A straight edge is only as flat as the grain of the material. Maybe granite would be a better material. Add a small amount of grit and water between two pieces of granite and rub back and forth and in a circular motion and viola, straight edge.