You can divide a foot in half, thirds, quarters, sixths, eighths, etc and the divisions will all end up exactly at a mark on your ruler or tape. Most of these can be subdivided again and again because the ruler is marked in 1/2, 1/4, 1/8, and 1/16 divisions, rather than powers of ten.

You can add any kind of divisions in the ruler, not matter what unit you use. It's just a historical curiosity.

Metric rulers just don't use those because there is tradition of using different fractions. In metric you typically use: 1, 3/4, 6/8, 5/8, 1/2, 3/8,1/4, 1/5, 1/8, 1/10, 1/20, 1/25, 1/50, 1/100...

I've never actually seen a "metric" ruler with non-decimal divisions, but that would work.

People go the other way too--some American machinists work in 'mils' (=1/1000 of an inch), since one often wants (and can achieve) that kind of precision.