Xiaobian takes you to the stationery. Does the mysterious ruler have a mysterious function? How to use the serrated edge of the ruler?

I walked into the stationery store and wanted to buy a ruler. Oh, my God, there are so many kinds! Pick them up handfuls and compare them. These different types of rulers seem to have their own advantages, but some functions are unclear like a mystery. For example, what is the serrated edge of a ruler for? When will it be used? (There are really a hundred question marks in my heart)

For all kinds of rulers in stationery stores, I can't help admiring the manufacturers who make rulers ... After reading it, there are probably the following types of rulers with dazzling array of things and a little "mystery" function:

Folding rulers

Photography: Wang Zining

Compared with the general 12cm and 15cm, usually folding rulers are 3cm, which is half the length when folded. All I know is that it seems easy to break the ORZ

maze ruler

Image source: from the Internet

I wonder if you have seen this maze ruler? I think this design is really mysterious. The ruler is used to draw lines (reasonable), the ruler is used to measure the length (reasonable), and there is a maze on the ruler (? ) serrated ruler

Photography: It is actually very common for Wang Zining

ruler to have serrated edges (maybe if you take out the ruler in your pencil case), which is a classic, but it is still puzzling. What mysterious function does the serrated edge of ruler have?