The most common function of a key is to prevent relative rotation of a shaft and the member to which it is connected, such as the hub of a gear, pulley, or crank. Many types of keys are available, and the choice in any installation depends on such factors as power requirements, tightness of fit, stability of connection, and cost. For light power requirements a setscrew may be tightened against the round shaft or against a flat spot on the shaft. For most requirements a positive connection, such as by a key, is necessary. A setscrew is frequently used to seat the key and to prevent axial motion.