The common element among screw fasteners is their thread. In general terms, the thread is a helix that causes the screw to advance into the work piece or nut when rotated. Threads may be external (screw) or internal (nut or threaded hole). Thread forms originally differed in each major manufacturing country but after World War II were standardized in Great Britain, Canada, and the United States to what is called the Unified- National Standard (UNS) series. The international standards for threads using metric dimensions are available now, and see the attached table. Both UNS and ISO threads are in general use in the United States. Note that the ISO defined threads are not interchangeable with UNS threads because of the unit conversion. However, both UNS and ISO share key geometric features, as illustrated in the figure shown above. Both use a 60° degree included angle and define thread size by the normlal outside (major) diameter d of an extemal thread. The thread pitch P is the distance between adjacent threads. The crests and roots are defined as flats to reduce the stress concentration from that of a sharp comer. The specifications allow for rounding of these flats due to tool wear. The pitch diameter dp and the root diameter dr are defined in terms of the thread pitch p with slightly different ratios used for UNS and ISO threads.

If a threaded rod as shown in the figure is subjected to pure tensile loading, one might expect its strength to be limited by the area of its minor (root) diameter, dr. However, testing of treaded rods in tension shows that their tensile strength is better defined by the average of the minor and pitch diameters. A tensile-stress area A' is defined as s defined as

For more information, review the document: ANSI/ASME Standard B1.13-1983 (R1989), American National Standards Institute, New York, 1989.