These types of fittings have two types of threads: male or female. Male fittings feature their threads on the outside and female fittings have threads on the inside.

When National Pipe Tapered (NPT) male and female counterparts are threaded together, the tapered threads deform which applies pressure on the couplings ultimately making a tight seal. Tapered threads can be imprecise and the threads require some sort of sealant or tape to insure a leak free connection.

National Pipe Straight Mechanical (NPSM) is found on pipe fittings that have a swivel nut. There is a 30° ball seat on the female thread that seals against the 30° chamfer on the male thread.  A NPT male will also seal on a female NPSM thread, provided the NPT male thread has a 30° chamfer.