American pipe thread standards — tapered (NPT) and straight (NPS).
NPT is tapered at 1:16 and seals metal-to-metal on the thread flanks with a sealant. NPS is parallel and seals on a gasket or O-ring against a shoulder. They share the same basic thread form and the same nominal sizes, so the designations look alike and the fittings behave completely differently.
NPTF is a dry-seal variant where the crest and root are controlled to interfere, allowing a seal without compound.
Threading an NPS fitting into an NPT port. It will go part-way in and then leak, which is worse than not fitting at all.
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