Pipe nipples and adapters are the connecting and transitioning fittings of any threaded piping system — they bridge gaps between other fittings, change thread size or type, and allow flexible field assembly of complex distribution geometries from a relatively small inventory of standard parts.
A hex nipple is a short pipe segment with male threads on both ends and a hex grip in the middle, used to join two female-threaded fittings together with a clearly-defined tightening surface. Close nipples are even shorter — the threads at both ends meet in the middle, with no smooth section between them — used where two fittings need to be installed essentially back-to-back with minimal space between them. Hex bushings have a male thread on the outside and a smaller female thread on the inside, used to reduce a larger threaded port to a smaller one within a single fitting.
Thread adapters connect between different thread standards. The most common transition in international work is between NPT (American National Standard Pipe Thread) and BSPT (British Standard Pipe Tapered) — the two threads have different angles (60° vs 55°) and different pitches, so they cannot be directly engaged without a proper adapter. NPT-to-BSPP (parallel) adapters also exist for connecting to instruments and equipment using the parallel BSP standard for O-ring or gasket sealing.
Male-to-female and female-to-male adapters change the gender of a thread connection without changing its size — useful when fittings need to be installed in a sequence where the existing thread orientations don't directly mate.
All of these fittings follow the same material and thread standards as other brass pipe fittings: C36000 or C37700 brass for industrial use, lead-free brass for potable water, 3000# forged brass for high-pressure service, and NPT or BSPT threads depending on the regional market.