EXCELLENT INFORMATION! :thumbup:Neil Hochstedler said:NPT means National Pipe Taper. NPS stands for National Pipe Straight. It's not tapered and is used mostly in electrical fittings.
If you want to know more about threads and threading, I suggest a Machinery's Handbook, used in an older edition and cheap through online used book services like biblio.com. Lots more info on threads, screws and bolts (learn the difference between them) and gears, press fits, slip fits, metallurgy, etc, etc.