Looking at an early caliper, you'll notice that it kinda looks like a tomahawk and has a scale down it's length. More about the scale later. On the bottom left, the "blade of the hatchet" are the jaws of the caliper for measuring outside dimensions - think of a tube. This would measure the outside diameter. The "pike" on top of the hatchet is used for measuring inside dimensions . Again, think of a tube's inside diameter. Looking all the way to the right...I know it's hard to see...but there's a little "tail" sticking out of the center of the handle. That will measure the depth of a step or some other similar feature. That little button on the top near the "pike" is a locking screw. The one just below it is a thumb button.