4. Engineer's Scale

A traditional Engineer's scale is six-sided, Figure C-11, with a division scale on each side.

Figure C-11
Engineer's Scale

The scales are 10, 20, 30, 40, 50, and 60 divisions per inch. Each scale can be used to represent a number of different map scales.
For example, the 10 scale can be used as 1 in = 1 ft, 1 in = 10 ft, 1 in = 100 ft, etc.
It can also be used for ratio scales, 1:1, 1:10, 1:100, etc.
Traditionally, maps use a nominal scale based on 10-20-30-40-50-60 to allow direct measurement without converting. This is less prevalent in digital mapping, but still a consideration when plotting a digital map.