Kk I looked it up
It literally is a straight thread version of pipe threads
Which is why it usually threads a turn or two with pipe fittings before jamming
Copy and pasted from internets:
Straight Pipe Threads for Mechanical Joints, NPSM and NPSL ANSI/ASME B1.20.1, American Pipe Threads Standard.
NPSC = National Pipe Straight Coupling.
Also generically referred to as "IPS Threads" or "Iron Straight Threads".
Note that straight pipe threads, unlike tapered pipe thread do not provide any sealing function. Typically, a design that utilizes a straight pipe thread will incorporate a sealing features, like an oring to facilitate fluid sealing
-s
