AN and NPT fittings

great chart and a little history!! :)

Since Geocities is un-reliable at best, here's the article text.

AN" Thread Sizes
AN sizes, originally developed for use by the U.S. Armed forces ("A" for army and "N" for navy), describe the outside diameter (O.D.) of tubing in 1/16-inch increments. For example, an AN 2 fitting will fit a tube with an O.D. of 2/16", or 1/8", while an AN 8 fitting will fit a tube with an O.D. of 8/16", or 1/2". Because the actual thickness of tube walls can vary from brand to brand, the inside diameter of a tube is not used as a reference. You will also find the dash (-) symbol or the word "dash" itself used in conjunction with AN sizes. A "dash six" fitting translates to AN-6.

Each AN fitting has an established thread sizing. The following chart shows the relationship between AN size, tube O.D., and SAE thread size:

"NPT" Thread Sizes
NPT sizes (National Pipe Taper) are the most commonly used fitting sizes for general plumbing, piping, and tubing use; not quite as popular as AN for automotive use, but still very common. While AN fittings depend on the outside diameter of a tube for sizing, NPT fittings depend on the interior diameter (I.D.) of the fitting itself. The following chart shows the each size's thread-per-inch count, the I.D. of the fitting, and the AN fitting size with the closest-matching I.D.(inside dimension).