This is the vernier caliper..
Open the jaws of the caliper and position them on both sides of the piece you are measuring.
Push the jaws firmly against the workpiece
Lock the clamp screw so that jaws don't move
On the vernier scale is a small number 0. Look at how many inch divisions it is past on then at scale.
See how many smaller number divisions the small 0 has gone by.
How many smaller divisions has the small number 0 gone past the last numbered division, this number multiple by 25 is how many hundredth of inches.
Determine which division line on the vernier scale best lines up with a division no the bar scale .