AN vs NPT info?

NPT is pipe thread, is tapered, and is "called out" by the NOMINAL inside dia, which is not really true. In other words, 3/8" NPT is "roughly" 3/8 ID. If you are trying to figure out what actual size pipe thread you are dealing with,

This page:

http://www.plumbingsupply.com/pipethreadsizing.html

has a chart showing pipe thread OD sizes

Pipe thread is also often referred to as #6 for 3/8, as example. The "number" sizes are simply a way to relate in 16's of an inch, mainly for shelf stocking and inventory.

1/8 pipe no2, 1/4 pipe no4, etc, etc

Also, do not confuse AN sometimes called by hydraulics guys as JIC, which has a 37* flare angle,

with SAE, your common brass fittings, which has a 45* flare angle. some sizes of the two will interchange, but at least no6 and no12 will not screw together.

In other words, some sizes of the two series, AN (JIC) and SAE will actually screw together, and some brands sell them interchangeably as to the sizes that WILL fit together.