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NPT stands for National Pipe Taper.
If a specific air fitting is manufactured to the NPT standard, it will have dimensional compatibility with other NPT threaded fittings of all other manufacturers. Which is another way to say that all NPT fittings are interconnectible.
One manufacturers fitting can be replaced with the same NPT size fitting of another manufacturer, and both - assuming they are the same size - will fit the same threaded hole in your air cylinder, actuator, air tool handle, valve...or whatever the application is.
Visit your favorite hardware or plumbing store and ask for a 1/4" fitting. The fitting they will show you will likely be a 1/4" NPT. You will look at the diameter of the thread and say to yourself, this ain't no 1/4" fitting. It will be. The actual diameter of the 1/4" NPT thread will be just over 1/2".
The size of an NPT fitting, teh 1/4", the 3/8", 1/2" and so on are reference sizes. The actual size of each is much larger than the fraction size they are supposed to be.
On this chart I have listed the NPT thread size first, and then what that thread size is in actual (decimal) inches, and then the closest common size fraction to that NPT size.
Compare that result to the fractional sizes on the chart to help guide you to the right male-threaded NPT fitting size to use.
NPT, as an acronym for National Pipe Taper, and is an industry dimensional standard. The threads of different fittings with the same NPT size will connect, but that does not mean that all of the fittings will necessarily look the same.
You need to identify the...
There are certainly much larger NPT sizes. The sizes listed above are the commonest fittings in industrial compressed air applications.
To aid in the thread penetration, and to help render the thread connection leak proof, it is common for folks to select Teflon tape as a sealing aid. They coat the male thread with the tape, allowing those male threads to turn deeper into the female boss than would be possible without the thread, and improving thread-to-thread penetration distance and the resulting tighter sealing.
Since compressed air flows very rapidly, sometimes pieces of the Teflon tape can break off in the air turbulence inside the fitting and create problems downstream by lodging in other air components. For example, pieces of tape can lodge in smaller air orifices and block them, or prevent another sealing surface from closing properly, actually generating a leak.
Yet Teflon tape is convenient, inexpensive and relatively easy to use. If you do use it, just be careful to keep the tape wound well back from the end of the fitting thread, and do not let the Teflon tape overlap air passages when the fitting is threaded into the hole.
Wind the Teflon tape onto the male threads against the thread, so when the fitting is threaded into the port, the Teflon tape would tend to tighten on the threads rather than loosen.
Like the Teflon tape, this sealing compound allows the NPT threads to seat further into the boss thread, allowing greater thread-to-thread contact inside, and filling micro-voids and unevenness in the threads to improve the seal. This is a nice feature of modern air fittings, and is both readily available and well accepted.
Elbow fittings are those that allow the supplying line to connect to the air fitting tangentially, out to one side, rather than straight in the end.
To make orientation of the air line easier, modern elbows are swivel type. After the thread is screwed tight into the port, the air line connection can be rotated to allow the air line to align in the most convenient direction.
If the elbow fitting is to be swiveling continuously, by a reciprocating part or some such on a machine, install a purpose-designed rotary union , NOT the swivel elbow fitting.
For example, rotary unions are commonly used on index tables, where there is continuous rotary movement of the air supply line to the table.
I recommend you select a quality built Rotary Union, one with ball bearings in their construction to allow easy, continuous rotation, one that has the proper seals necessary to ensure reliable, often high speed, continuous rotation without fitting failure or air leaks.