Threaded pipes can provide an effective seal for pipes transporting liquids, gases, steam, and hydraulic fluid.  These threads are now used in materials other than steel and brass, including PTFE, PVC, nylon, bronze, and cast iron.
The taper on NPT threads allows them to form a seal when torqued as the flanks of the threads compress against each other, as opposed to parallel/straight thread fittings or compression fittings in which the threads merely hold the pieces together and do not provide a seal. The various types of pipe threads are designed for use both with or without thread sealants, as [https://casadasvalvulasmg.com.br/roscas-bsp-e-npt-aplicacao-de-cada-uma/ particular applications demand]. The sealants, where used, are typically thread seal tape thread sealant or a thread seal paste pipe dope (sometimes similar in composition to plumber's putty).
Because the thread body is tapered (), a larger diameter keeps compressing into a smaller diameter and finally forms a seal (no clearance remains between the crests and roots of the threads because of the taper). This means that NPT fittings should be burr-free and lubricated using a lubricant like lubricating paste or thread seal tape. The use of tape also helps to limit corrosion on the threads, which otherwise can make future disassembly nearly impossible.
Commonly used sizes are , , , , , 1, , , 2, , 3, 4, 5, and 6 inch, as pipe and fittings by most US suppliers. Sizes smaller than  inch are occasionally used for compressed air, while sizes larger than 6 inches are often joined by other methods.