Scale rule

A scale rule is a specialized ruler, used in creating and reading draftings and other scale drawings. Two common styles are available: architect's and engineer's. Specialty scale rules are also available, such as those with scales specific to model railroading. Often, each edge of the rule carries two scales, one twice the scale of the other, and read from opposite ends of the rule.

While many scale rules are flat, appearing at quick glance like a standard rule (ruler), those with a triangular cross section allow the inclusion of more scales.

Architect's
An architect's scale rule is marked with the scales most commonly used in architecture: 3, 1-1/2, 1, 3/4, 1/2, 3/8, 1/4, 3/16, 1/8, and 3/32

Engineer's
An engineer's scale rule is marked with the scales most common in engineering drawings:

Reading a scale rule
Regardless of the type and scale, reading a scale rule is done the same way.