Nominal Pipe Size (abbreviated NPS) is a term for American standard pipe sizes widely used in the North American industrial trades, but does not describe specific thread forms. Confusingly, National Pipe Straight (also abbreviated NPS) is the name of a specific type of thread form described in American National Standard Pipe Thread standards. Just to be clear they are not the same, one NPS describes a standard for specific sizes of pipe and the other NPS describes a specific type of screw threads used on the ends of pipe.

Historically, many different screw threads have been used on SCUBA cylinder outlets and valves. Using the national pipe thread standards, the proper technical name of the parallel straight threads commonly used to mate modern SCUBA cylinders and valves for the North American market is 0.750-14 National Pipe Straight Mechanical, typically abbreviated as 0.750-14 NPSM. It is frequently also written as 3/4 NPS or 3/4-14; for the purposes of description with SCUBA cylinder and valve applications the terms may all be considered to reference the same threads: 0.750-14 NPSM has a 60° thread angle, a pitch diameter of 0.9820 to 0.9873 in (24.94 to 25.08 mm), and a thread pitch of 14 threads per inch (1.814 mm). The same thread form is also described in a variety of other standards such as ANSI, ASTM, and ISO but those other standards are very rarely referenced in the North American SCUBA market.

See our article SCUBA Valves for more information.