Boley Gauge

This product is also known as Boley Caliper and is manufactured in stainless steel and finished in satin. A Boley Gauge has a fixed scale and a sliding scale.
Begin by looking at the fixed scale where the boldface numbers are in cm. The 1 and 2 on the fixed scale represent 1cm (10mm) and 2cm (20mm) respectively.

The zero mark is the first tick mark on the Boley Gauge sliding scale. The zero mark determines the whole mm amount depending on where it is located relative to the fixed scale. For example, if it’s between the 11mm and 12mm tick mark on the fixed scale, it’s 11mm. If it’s between the 12mm and 13mm the whole mm amount is 12mm. Remember: the zero tick mark on the sliding scale does not count as 1.
As you look along the Boley Caliper sliding scale find which tick mark on the gauge sliding scale most closely lines up with a tick mark on the gauge fixed scale. That tick mark is going to represent your tenths of a mm.