How do you know a line is straight? How can you check in a practical way if something is straight - without assuming that you have a ruler? How do you detect that something is not straight?
If you fold a piece of paper the crease will be straight — the edges of the paper needn’t even be straight. This utilizes mirror symmetry to produce the straight line.
Carpenters also use symmetry to determine straightness — they put two boards face to face, plane the edges until they look straight, and then turn one board over so the planed edges are touching.They then hold the boards up to the light. If light passes between the boards the edges are not straight.
Are there other ways to determine if a line is straight?