Threaded pipe
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A threaded pipe is a pipe with screw-threaded ends for assembly.
A threaded pipe plug; note that the threaded portion is tapered and so touches the calipers at only two points.
Nipple (plumbing)
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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.
Nipple