1 Plastic Vernier Calliper Gauge, measures beads and items upto 80mm, 3-1/4” .
Use to measure outside diameter with large jaws, inside diameter with small jaws and depth with end probe, calibrated in both metric and imperial units.
In metric the fixed scale divisions are whole millimetres, while the slider scale is tenths. 

In our example (detail photo) the size 12.7mm is measured by reading the ruler at the first mark of the slider '12' then determine where the scale marks best align on the two parts and adding the number of slider divisions '7' as the tenths, to make 12.7mm.

It may be easier to learn how use in the video below