A pp compression valve is a type of coupling used to connect two pipes or a pipe to a fixture or valve. It consists of three parts... the compression nut, the compression ring, and the compression seat. As you can see in the diagram at the left, the nut is slid onto the pipe, followed by the compression ring.The pipe is slid into the fitting (in this case a toilet shutoff valve) and the nut is tightened down. As the nut is tightened, the compression ring is pressed into the seat, causing it to compress against the pipe and the compression nut, providing a watertight connection. Usually, that is.Though the primary advantage of compression fittings may seem to be in their ease of assembly, their real advantage is in their ease of disassembly! If you look throughout your home, you will see that the use of compression fittings is generally limited to appliances and fixtures that will... given enough time, normal use, and wild beatings with a hairbrush... wear out and need to be replaced. Look under your bathroom vanities, behind your toilets, under your dishwasher or behind your refrigerator... and you will see compression fittings in flagrante delicto!In fact, the now ubiquitous use of compression fittings in homes conveniently corresponds with the advent of do-it-yourself , user-friendly fixtures and appliances. After all, if you want to sell faucets to folks with limited skills, you want to make their installation as painless as possible. Ergo, compression fittings!But compression fittings have a dark side, and may leak if not used properly. Here are my rules for the use of compression fittings. If you don't follow them, you may regret it!