A bore gage measures inside hole diameter with high accuracy. The controller can either turn a knob or release a trigger to extend pieces outward until the wall perimeter is touched by each contact. A bore gage is usually chosen by categorizing its precision measurement range, depth, accuracy requirements, and the number of contacts. A bore gage can have interchangeable measuring heads, different diameters, and extensions for depth, depending on their usage. A bore gage can also have electronic displays, micrometer-type vernier scales, or a dial that is similar to an indicator.