There are many types of the screwed joints, here is a brief introduction for some screwed symbol:

NPT, PT, G, BSPT all belongs to pipe thread.

NPT: National (American) Pipe Thread. NPT is American system taper pipe thread, used for sealing. It belongs to 60 degree taper pipe thread in American system, used in North America. For China standard, please refer to GB/T12716-1991.

PT: Pipe Thread. PT is 55 degree sealing taper pipe thread, it is a kind of Whitworth threads, commonly used in European and British Commonwealth of Nations. It is frequently used in water and gas pipe industries, regulated taper is 1:16. For China standard, please refer to GB/T7306-2000.

G: Guan (Chinese Pinyin for “pipe”). G is a general name for pipe thread, it’s 55 degree non thread sealing, one of Whitworth threads. G means straight thread.

BSPT: British Standard Pipe Thread. The thread angle is 55 degree, taper is 1: 16.

Metric thread is signified by thread pitch, American and British thread is signified by threads per inch, that’s the biggest difference between them.

Metric thread is 60 degree equilateral thread form, inch thread is isosceles 55 degree thread form, America thread is 60 degree.

Pipe thread mainly used for connecting pipeline, its inside and outside thread fit closely, in forms of straight pipe and taper pipe.