Also called "suction hose couplings," these are threaded couplings used for suction or discharge of water or other fluids. Standard threading is NPSM (National Pipe Straight Mechanical). 1-1/2" and 2-1/2" are available in NST thread (American National Fire Hose Straight Thread). NST does not interchange. Pin lugs are on all sizes of the female end. Sizes 2-1/2" through 6" also have pin lugs on the male end. Fittings seal on a washer that sits in the female end. Replacement washers are available.