Finding the right fit of PVC can be a hassle when you are working on a project. The biggest issue is the sizing is not always accurate. If you look at a piece of pipe, you will see an ID number on it. ID is the inside diameter of the pipe, which means that when looking for pipe, don't mistakenly measure the outside diameter for matching.
So, if it says "1", that means it is supposed to be approximately one inch in diameter. The actual measurement of the ID can be anywhere from .95 inches to 1.09 inches. The reason is that when PVC is produced, it is difficult to make exact fit. So those measurements are within what is known as "nominal" range.
After identifying the proper size of PVC pipe you need, you have to find the appropriate fitting. There are plenty of options available for different needs. Here are a few examples.
Coupling
The simplest type of fitting is called a coupling. It is a very short PVC tube that has a socket on each end to fit the longer pipes. If you need to fit two pieces of pipe together that are separate dimensions, you would buy what is known as a "reducing coupling".
Union
Similar to a coupling is a union, as it is designed to fit two pieces of pipe together. However, the difference is that a union is designed to disconnect the two pipes easier. The alternative name for this is a "quick disconnect".
Street Fitting
Whenever you want to attach two fittings to one another, you have two options. One is to take a short piece of pipe and attach it to the slip sockets in both fittings. The alternative is what is known as "street fitting" or "spigot fitting", which is where one of the fittings is designed specifically to fit into another fitting.
Threading
Some PVC fittings have threading on them, similar to what the head of a screw would have. These come in two varieties "male" and "female". As the names imply, the male threading is has external threading that fits neatly into a female threading adapter. If you have female threading, you will need a male adapter. All of the fittings will have a slip sockets on the other end.
Reducer Bushing
A reducer bushing will allow you to fit a piece with a slip socket onto a pipe of a different size. Reducer bushings come with threading or without, depending on your needs. So, for example, if you have a fitting with male threading that you need to fit another piece of pipe on it, you will need a reducer bushing with female threading.
Nipple
Nipples are pipes that have male threading on both sides. A close nipple is just one long male thread. A reducing nipple, also known as an adapter nipple, will have male threads of two different sizes on each side.