Sometimes "special" diameter and pitch combinations (UNS) are used, for example a major diameter with 20 threads per inch. UNS threads are rarely used for bolts, but rather on nuts, tapped holes, and threaded ODs. Because of this UNS taps are readily available. Most UNS threads have more threads per inch than the correlating UNF or UNEF standard; therefore they are often the strongest thread available. Because of this they are often used in applications where high stresses are encountered, such as machine tool spindles or automotive spindles.