How was the first ruler/tape measure made to be straight and have accurate spacings between numbers?

Take a piece of paper and fold it in half, then fold that in half and so on. Now you've got equally spaced folds. This is the reason why measurements were first not in decimal units, i.e. in 1/2, 1/4, 1/8 etc. For the straight part, just fold a paper length wise and you have a straight edge.How was the first ruler/tape measure made to be straight and have accurate spacings between numbers?
It has been known for thousands of years (since Greeks at least) that if a group of parallel lines are made on a surface - say by laying a piece of wood or stone of constant width down on one line and scribing along it for the next line - then a rod or stick can be laid across the lines at an angle to make a perfectly evenly spaced ruler.


With a little care, a round rod with a disk on the end can be rolled on a flat surface and lines scribed along it as it is rolled 1/4, 1/5, 1/6, 1/8, 1/10, 1/12, 1/16 revolution using squares, pentagons, hexagons, etc, constructed on the disk.


How was the first ruler/tape measure made to be straight and have accurate spacings between numbers?
Simple. The straightness was measured against a steel rule.