Notice About Bundle Packs:Please understand our bundle packs share from a communal inventory. A buyout of one pack type will consume inventory of other packs. Due to limitations, our system has a delay in updating other pack stock quantities. Thank you graciously for understanding. National Pipe Taper (NPT) are a standardized fitting that is commonly used for joining pipe. Due to the tapered threads (wider diameter at the base of the threads, similar to a trapezoid) the male fitting will tighten the further it is threaded into the female. NPT follows the technical standards provided by the American National Standard Pipe Thread standards, often called national pipe thread, which is often where the confusions with NPT arises, as NPT is specifically an acronym that referes to tapered pipe. National Pipe Straight (NPS) are straight thread that is sized to be compatible with common water heater elements and should be compatible with most NPS straight threads. Determining NPT/NPS Size:Sizing NPT/NPS is based on the nominal pipe diameter, and not the ouside diameter (OD) of the threads. For example: A 1/2" NPT male has an outside thread diameter of approximately 3/4", but as stated the correct NPT size is 1/2", including the counterpart female. Notice: While NPT and NPS threads may be joined together, their threads may not form a proper seal, be loose fitting, or otherwise not an optimal connection.