 Though you may consider them a tool for mechanical engineers, calipers are surprisingly versatile for makers of all types. By gently closing the larger set of jaws, one can measure an object's outer diameter. Just make sure the flat sides are flush against the object's surface. For a more careful adjustment, you can move the jaws around the object using the thumb wheel. Similarly, fully spreading the smaller set of jaws within an opening lets you measure inner diameter. And you can measure depth by holding the flat end of the calipers against a surface and rolling the thumb wheel to extend this metal post down into the bottom of a cavity. Most digital calipers even let you choose between measuring in inches or millimeters. Personally, I enjoy millimeters.