All calipers, whether vernier, dial or digital, include the same basic components. They have two jaws. The fixed one is the reference jaw and the sliding one makes the measurements. All calipers incorporate a little “hump” where your thumb goes to help move the measuring scale back and forth. Some incorporate a thumbwheel to help improve repeatability by trying to maintain constant gaging pressure (the wheel disengages when the sliding jaw comes into contact with the part).