Engineers and machinists use the term scale, denoting that often the measurement on a drawing has a scaling factor applied, e.g. 1" = 1'-0". This company makes machine tools or something like (didn't research the site, didn't need the 3" scale). Allisohm is right that it's really a ruler. A scale, properly, is usually a triangular prism with two scaling factors per face. Usually, an engineer or drafter has two scales: engineering and architectural. The engineering scale is larger, with 1"=10', 1"=20', etc. The architectural scale is smaller, with 1/4"=1'-0", 3/8"=1'-0", etc. In my thinking, the machinists have been trained to use the word scale and now feel funny if they call this a ruler. My dad had a couple of scales when I was a kid, and I ruined one using the triangular edge to break up rock candy. I guess he never noticed.....