The increments on a radius gauge scale are similar but not the same as those on a standard ruler, in that the numbers increase exponentially as you go up the scale.
Toward the 6″ radius end of the scale, the section between 6″ and 12″ of radius, each line represents .10″ of radius change as indicated by the .10″ number above these radius measurements. At the other end of the scale, between 96″and 144″ of radius, each line represents 4″ of radius change. As examples, the arrow located toward the left end of the scale points to the radius measurement of 8.60″ and also the pointer further up the scale indicates a radius of 54″. The metric scale is setup similarly .
The format for both IRG-3600 and ORG-3600 are identical except for slight changes in the positioning of the lines.