British Standard Pipe

British Standard Pipe (BSP) is a set of technical standards for screw threads that has been adopted internationally for interconnecting and sealing pipes and fittings by mating an external (male) thread with an internal (female) thread. It has been adopted as standard in plumbing and pipe fitting, except in North America, where NPT and related threads are used.
Types.
Two types of threads are distinguished:
These can be combined into two types of joints:
Thread form.
The thread form follows the British Standard Whitworth standard:
Pipe thread sizes.
At least 41 thread sizes have been defined, ranging from 1⁄16 to 18, although of these only 15 are included in ISO 7 and 24 in ISO 228. The size number was originally based on the "inner" diameter (measured in inches) of a steel tube for which the thread was intended, but contemporary pipes tend to use thinner walls to save material, and thus have an inner diameter larger than this nominal size. In the modern standard metric version, it is simply a size number, where listed diameter size is the major outer diameter of the external thread. For a taper thread, it is the diameter at the "gauge length" (plus/minus one thread pitch) from the small end of the thread. The taper is 1:16, meaning that for each 16 units of measurement increase in the distance from the end, the diameter increases by 1 unit of measurement.
These standard pipe threads are formally referred to by the following sequence of blocks:
Threads are normally right-hand. For left-hand threads, the letters, "LH", are appended.
Example: Pipe thread EN 10226 Rp 
The terminology for the use of "G" and "R" originated from Germany ("G" for "gas", as it was originally designed for use on gas pipes; "R" for "rohr", meaning "pipe".)
ISO 7 (Pressure Tight threads).
The standard ISO 7 - Pipe threads where pressure-tight joints are made on the threads consists of the following parts:
ISO 228 (Non Pressure Tight Threads).
The standard ISO 228 - Pipe threads where pressure-tight joints are not made on the threads consists of the following parts: