Today, we are going to talk about short lengths of pipe threaded to attach fittings at both ends. Ready? “Nipple." There, I said it. Get your giggles out of the way. The fourth grader in all of us loves a word that sounds like it could be dirty, but isn't.
Funny Name
Leaving aside that many people aren't immature as I am, using the word “nipple" to describe a short piece of pipe is surprising to people who don't deal regularly in plumbing supplies. And it isn't entirely clear where and when people started using the term “nipple" to refer to pipes. I couldn't find the first use of first use of pipe nipple in plumbing or engineering. The best I could do was find the etymology of the word from www.etymonline.com. It dates from at least the 1530s and comes from a root meaning "bill, beak, snout" - literally “small projection." 
Types of Pipe Nipples and Their Uses
But no matter what the basis for the word, it is now in common use in plumbing. And what's more, there are several different types/subsets of pipe nipples to contend with. A short list includes: close nipple, hexagonal nipple, long hex nipple, reducing nipple, hose nipple and welding nipple. I'm sure there are more, but we'll stick with those for now. In its most basic form, a nipple is a short length of pipe with male pipe threads at both ends for connecting other fittings. You may also see them referred to as “barrel nipples". Generally speaking, there is a short distance of unthreaded pipe between the two threaded ends, depending on how far apart you need the attached fittings to be. When there is no bare pipe (read “unthreaded") between the two connecting ends, the part may be called a “close nipple" or a “running nipple". In that case, connected fittings come close to touching one another and very little of the nipple can be seen. Although some constructions require such tight placement, close nipples can be difficult to work with, since unscrewing them requires holding tight to part of the threaded area. That can damage the threads. Plumbers who need to use them often invest in an internal pipe wrench (also known as a “nipple wrench"), which expands inside the pipe to hold it in place without damaging the threads. In cases where you can spare a little space between threaded ends, you can use a “hexagonal nipple", which has a hexagonal section in the middle. It functions like a nut that can be gripped by a normal wrench, providing a greater mechanical advantage than normal rounded pipe. A hexagonal nipple with more distance between the threaded ends is called, no surprise, a “long hex nipple". For projects which require a change in pipe dimension, you can buy a “reducing nipple." Sometimes called an “unequal nipple", it takes a female fitting with a larger connection and attaches it to a smaller one. Care should be taken when using these parts since a reduction in pipe diameter can mean more pressure and greater flow rate in the smaller pipe/fitting. The nipple usually has a hexagonal center, although not always. There is a specialty nipple called a “hose nipple" for processes which require connection to tubing. That fitting features a male threaded connection on one end and a hose barb on the other. The hose barb may be the same size as the pipe connection or it may be reduced. The final variant we will talk about is the “welding nipple," which has a threaded connection on one end and normal cut pipe at the other. As the name implies, this part is designed to be welded to piping, a fitting or a tank. The unthreaded end of the pipe provides more surface area for the use of solder or other welding materials to make the connection stronger. The benefit of the welding nipple is that once the unthreaded end is connected, you can connect pipes or other fittings to the threaded end much more easily.