Bore gages measure inside hole diameter with high accuracy. The operator turns a knob or releases a trigger to simultaneously extend the contacts outward until the wall is touched by all contacts. Bore gages are selected by measurement range, depth, accuracy requirements and number of contacts (two or three). Some are available with interchangeable measuring heads, different diameters, and extensions for depth. They can have electronic displays, micrometer-type vernier scales or a dial.