It made it easy to calculate relative measurements when you needed to make enlargements or reductions.It looks like a circular slide rule. The smaller inside wheel represents the size of the original, while a larger outside wheel measures the size of the reproduction. Each wheel is marked in a gradually increasing scale from 1 to 100 inches. The two wheels rotate independently, held concentrically by a grommet.
A small window marks the percentage enlargement or reduction. If you set it to a 24 percent reduction, all the relative measurements between the two wheels will be held at that relationship.
Knowing such measurements was necessary for making photographic enlargements, planning original art for a given print application, or specifying type. Although it may be obsolete for many of its original graphics art uses, it’s still a useful tool for modelmakers, craftspeople or anyone who needs to scale something up or down by percentages.