Certain types of small tools and machine parts, such as twist drills, end mills, arbors, lathe centers, etc., are provided with taper shanks which fit into spindles or sockets of corresponding taper, thus providing not only accurate alignment between the tool or other part and its supporting member, but also more or less frictional resistance for driving the tool. There are several standards for self-holding tapers, but the American National, Morse, and the Brown & Sharpe are the standards most widely used by American manufacturers.

The name self-holding has been applied to the smaller tapers like the Morse and the Brown & Sharpe because, where the angle of the taper is only 2 or 3 degrees, the shank of a tool is so firmly seated in its socket that there is considerable frictional resistance to any force tending to turn or rotate the tool relative to the socket. The term self-holding is used to distinguish relatively small tapers from the larger or self-releasing type. A milling machine spindle having a taper of 3 1/2 inches per foot is an example of a self-releasing taper. The included angle in this case is over 16 degrees and the tool or arbor requires a positive locking device to prevent slipping, but the shank may be released or removed more readily than one having a smaller taper of the self-holding type.