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The length of the nipple is usually specified by the overall length with threads. It may have a hexagonal section in the center for 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. A close nipple can only be unscrewed by gripping one threaded end with a pipe wrench which will damage the threads and necessitate replacing the nipple, or by using a specialty tool known as a nipple wrench (or known as an internal pipe wrench) which grips the inside of the pipe, leaving the threads undamaged. When the ends are of two different sizes it is called a reducer or unequal nipple.
- ASTM A733-03 Standard Specification for Welded and Seamless Carbon Steel and Austenitic Stainless Steel Pipe Nipples.
- ASTM B687-99(2005)e1 Standard Specification for Brass, Copper, and Chromium-Plated Pipe Nipples.
- ASME B1.20.7 Hose Coupling Screw Threads, Inch. (Quote: The normal sequence of connections, in relation to the direction of flow, is from an externally threaded nipple into an internally threaded coupling)