Determined by the shape formed by the screw's flank and crest, some common types of screw threads include metric, unified, buttress screw, round and trapezoidal. Other important identifying features of a screw thread include its thread form, thread pitch, major diameter, pitch diameter and minor diameter.
There are two ways to describe the number of threads found on a screw. A screw's thread count, or TPI, measures its threads per inch along the fastener, based on inches from the American measuring system, while a screw's thread pitch measures the distance between each thread in millimeters, based on measurements from the metric system. Two popular types of fastener threads are spaced threads and machine screw threads. Spaced threads are designed to create threads in a pre-drilled hole, whereas machine screw threads are symmetrical to match an existing hole.