Taps are used to form threads or to chase (clean up) threads in holes drilled into metal. Depending on the application, a specific type of tap may be required. Hand taps are turned manually and include subcategories such as bottoming taps, plug taps, and taper taps. Power taps are used in Computer Numerical Control (CNC) milling, which is machine driven and automated. Taps are available as individual units, in tap sets, or as part of a tap and die set. Dies and die sets are used to cut, or clean up, threads on bolts and similar fasteners.