If you are measuring the outer edge of an object, like, let's say the head of an ushabti figure, you would open the jaws of the calipers and slide them carefully over the head of the ushabti. Read the sliding scales along either top or bottom or read the dial to see the size of the head. Care must be taken to assure that your calipers are straight. Even a slight angle throws off your measurements. Remember this is highly precise work and readings must be taken very, very carefully to assure proper dimension and accurate drawings.
Inner Edges Take a look at the top edge of the dial calipers.
Those are to take measurements of the inside of holes like on the opening of a jar. Put them inside the opening and carefully spread them using the thumb grip. They will widen simultaneously as the jaws open. Bear in mind that hand made objects will have variance unlike the machine made objects of our time. Depth Now look at the tail end of the dial calipers.