Nipples: A nipple is a fitting, consisting of a short piece of pipe, usually provided with a male pipe thread at each end, for connecting two other fittings. The length of the nipple is usually specified by the overall length with thread. It may have a hexagonal section in the center for a wrench to grasp (sometimes referred to as a “hex nipple”), or it may simply be made from a short piece of pipe (sometimes referred to as a “barrel nipple” or “pipe nipple”). A “close nipple” has no unthreaded area; when screwed tightly between two female fittings, very little of the nipple remains exposed.