In plumbing and piping, 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 or valves
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It may have a hexagonal section in the center for a wrench to grasp (sometimes referred to 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 the ends are of two different sizes it is called a reducer or unequal nipple.