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Nominal Pipe sizes do not match any of its actual physical dimensions
Two common pipe thread sizes exist, the tapered National Pipe Thread (NPT) and the straight National Standard Free-Fitting Straight Mechanical Pipe Thread (NPSM).
The tapered threads are for joining and sealing, the straight threads are only for joining.
Dry-seal thread (NPTF) allows for joining without sealants.
Female NPT threads may be designated as "FPT"
Male NPT threads may be designated as "MPT."
The pipe size is often written such as this 1/8"-27, with a hyphen between size & thread per inch, and pronounced as "one eighth twenty-seven"
Three less common threads exist, the Garden Hose Thread (GHT),
Fire Hose Coupling (NST) and British Standard Taper Pipe Thread (BSPT).
The NPT and NPTF threads are interchangeable with sealants such as PTFE tape.
None of the other thread standards are interchangeable.
