The dial calipers are the second generation calipers. The construction of the dial calipers is different from the vernier calipers, as there is a a dial indicator instead of the vernier scale. And the indicator is fixed to the moveable jaw and has toothed rack on the body of the unit. The dial is typically balanced (namely, can move in either plus or minus directions from zero) and is graduated in either inch or metric units.
Dial calipers can be used for either direct or comparative measurements. To make a comparison, first measure the reference dimension and set the dial indicator to zero. Then measure the compared dimension. The indicator will show how much the compared dimension varies from the original (plus or minus).
Another advantages of the dial calipers are jaws that can slide past each other to allow contact points or depth rod extensions to fit into narrow openings for small ID measurements.
For dial calipers, be particularly wary of dirt, which can accumulate on the rack, throwing measurements off and ultimately damaging the indicator. Store a caliper in its case. If it's going to be there for a while, apply a thin coat of oil to the jaws to inhibit corrosion.