Parallel Thread Pipe Fittings

An alternative to tapered threads in pipe joints is the use of parallel threads, similar to the threads
of machine screws and bolts. Since parallel threads are incapable of forming a pressure-tight seal
on their own, the sealing action of a parallel thread pipe fitting must be achieved some other way.
This function is usually met with an O-ring or gasket.
A common design of parallel-thread pipe fitting is the SAE straight thread, named after the Society of Automotive Engineers:

Sealing is accomplished as the O-ring is compressed against the shoulder of the female fitting.
The threads serve only to provide force (not fluid sealing), much like the threads of a fastener.