TO REMEMBER

1 - The metric system, divided into metric gross (M) and fine (MF), is the most widespread in France. The pitch is often expressed into 1/100 of millimeter (M6X100), and the thread angle is 60 °.
2 - The UN (Unified National) American system and its UNC derivatives UNC for a pitch konwn as "large", UNF for a "fine" pitch, UNEF for "superfine" and UNS for special ones. This system uses the measurement system in inches and fractions (1 " = 25.4mm) and the pitch is expressed in the number of threads per inch, where the thread angle is of 60 °.
3 - The BS (British standard) English system is divided into BSW for a "large" pitch, BSF for a "fine" pitch, and BSC for special ones. This system is very close to the UN system with usually the same pairs of diameters / pitch but with the difference in a thread angle of 55 °.
4 - Piping pitch, usually called "GAS" by the users are indicated in fractions of inch, but here the inch measures 33cm and is not even constant since 2 " = 59.6mm. It is here necessary to make a distinction between cylindrical and conical pitching, and between English BSP, BSPT (thread angle 55 °) and the Americans NPS and NPT (thread angle of 60 °) without speaking of their variations NPSC, NPSF, NPSM, etc...
5 - "Exotic" or "hybrid": PG or electric pitch. The diameter is a number without dimension, nor any obvious relationship to the real diameter, the pitch is the number of threads per inch,the thread angle is 80 °, BA for British Association where the diameter is expressed by a number and the pitch is metric.