There are many threads used in world wide according to local standard and fitting criteria some are UNF, BSPP, NPT, BSPT, Metric etc. Most commons thread are BSPT, NPT And BSPP to seal between to male female connection, NPT (National Pipe Taper Threads) and BSPT (British Standard Pipe Taper Thread) threads are very similar thread where in NPT thread tapered 60° while in BSPT it’s 55° to create solid seal connections. On other hand in BSPP (British Standard Parallel Pipe Thread)O ring or seal ring used in  between male and female thread connection to seal threads.