The Vernier caliper provides accurate measurements of inside diameter, outside diameter and depth by using opposing scales to provide a much finer measurement than would be possible with a simple rule.
The mm scale on the Starrett 125 is a retrograde vernier (the vernier scale is more widely spaced than the main scale) this makes it much easier to read and accurate to 0.05mm rather than the 0.20mm commonly found on direct verniers.
The method of use is exactly the same - just take the crude measurement on the main scale, then look for point where the two scales align and read off the parts of a mm on the vernier scale.
If you are an inches person who prefers 32nds and 64ths to decimal inches, the imperial scale reads in exactly the same way and will take you down to 128ths.
A lovely bit of 15th century technology that still serves its purpose very well.
Graduations Imperial: 1/128th
Graduations Metric 0.05mm
Measuring accuracy Inches +/- 0.001 inch at up to 4 inch, +/- 0.0015 inch at 4 inch to 6 inch.
Measuring accuracy mm +/- 0.025mm per 300mm
Supplied in a fitted, foam lined plastic case.