At least 41 thread sizes have been defined, ranging from ⁄ 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:
* the words, Pipe thread,
* the document number of the standard (e.g., ISO 7 or EN 10226)
* the symbol for the pipe thread type:
** G, external and internal parallel (ISO 228)
** R, external taper (ISO 7)
** Rp, internal parallel (ISO 7/1)
** Rc, internal taper (ISO 7)
** Rs, external parallel
* the thread size
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.)