The British Standard Pipe (BSP) is a family of standard screw thread types that has been adaopted internationally for interconnecting and sealing pipe ends by mating an external (male) with an internal (female) thread and has been adopted as a standard scale used in plumbing fittings, except in the United Stated where NPT is the standard used.
A list of 16 thread sizes are defined by the standards, ranging from 1⁄16 to 6. The size number was originally based on the inner diameter measured in inches of a steel tube for which the thread was intended, but in the modern metric version of the standard, it is simply a size number.