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 (data output options), micrometer-type vernier scales or a dial (similar to an indicator). The Starrett line of bore gages is extensive, with products available for a broad range of applications.