You put the object between the two lagest jaws, pull them snugly together around it and make your reading.
But
you can do other things with them. If you want to measure the size,
inner diameter, of a hole, there are ways to do that.The classic use of vernier caliper: measuring the diameter of
round things.
You put the object between the two lagest jaws, pull them snugly
together around it and make your reading.
But
you can do other things with them. If you want to measure the size,
inner diameter, of a hole, there are ways to do that.
. 
If you want to measure the inner diameter of these contact holes, you
use the small sharp jaws on the other side.
You pull them apart until they press against the sides of the holes,
and make your reading on the same scales as the other jaws.
In the bottom end of the caliper is a narrow tongue that sticks out as long as the distance between the jaws. You can use it to measure the depth of a hole.
Put the bottom ot the scale body against the
edge of the hole. Pull the jaws apart untill the narrow tongue presses
against the bottom of the hole.
Try to make a reading on this vernier caliper picture!
Answer: reading = 36,4 mm or 3,64 cm or possibly 36,45 mm or 3,645 cm
2 Simulation showing how to use a (vernier or digital) caliper | 3 animation of reading vernier scales