The proper connection may not be obvious, like when a straight NPS
female thread with a rubber gasket seals to a tapered NPT male pipe
thread. Some household examples are sink and toilet feed lines and
kitchen and shower spray hoses.
A similar apparent mismatch with the misleading name of "swivel" is
common in hydraulics. They have a compressed metal-to-metal seal that
swivels only before final tightening.
"A properly chamfered NPTF male will also seal with the NPSM female."