The angle cut between the threads, number of threads per inch, and presence or absence of a taper determine the thread type in gas line connectors. The American national standard for pipe specifies all pipe thread types used in plumbing.
The most common, general-use thread type is national pipe thread, or NPT. It has tapered threads with a 60-degree angle and is used for mechanical and low pressure air or liquid applications. It must be sealed using polytetrafluoroethylene, or PTFE, tape or pipe sealant. Dry-seal thread, known as NPTF, also has tapered threads with a 60-degree angle. It is used in applications where exposure to higher heat or pressure could result in pipe sealant failure. NPTF threads compress when tightened with a wrench creating a mechanical seal. NPT and NPTF pipe are interchangeable only with the use of PTFE tape or pipe sealant.
National standard free-fitting straight mechanical threads are not tapered, are used only in joining, and require the use of a washer or gasket to create a seal. Garden hose thread and fire hose thread are less commonly used and have coarse threads sealed by a gasket or washer. They are used for a quick connection to a valve without the use of a wrench. The British standard taper pipe thread is the international standard for joining steel pipe and has a 55-degree thread angle.