Basic explanation of the different threads and their uses What is the difference between "NH", "NST", "NPSH", "IPT", and "NPT" threads? "NH", or National Hose, thread is the thread that has become the standard thread used in firefighting today (Cal Fire and Municipal Fire Departments). The threads specified in NFPA 1963 are "NH". This thread was previously known as "NST" or National Standard Thread. "NPSH", or National Pipe Straight Hose, is slightly smaller in diameter and has more threads per inch than the same nominal diameter "NH" thread. "NPSH" thread is sometimes referred to as "IPT", or Iron Pipe Thread and is commonly used in Forestry applications (USFS). "NH" and "NPSH" threads are not interchangeable. "NH" and "NPSH" threads are straight threads and require a gasket for sealing between mating threads. "NPT", or National Pipe Tapered, is the thread most commonly found in plumbing applications. Being tapered, the seal between mating threads is made by squeezing the threads together. No gasket is used. "NPT" and "NPSH" threads are similar in diameter and threads per inch.