[krispyfi]

A ruler is called a "ruler" because it has rules.

> A ruler, sometimes called a rule, scale or a line gauge or metre/meter stick, is an instrument used to make length measurements, whereby a length is read from a series of markings called "rules" along an edge of the device.

https://en.m.wikipedia.org/wiki/Ruler

[rerdavies]

A ruler is called a ruler because you use it to MAKE rules. Fixed that for u, wikipedia. :-)