What is a compression key used for?

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	This type of wrench is used to tighten compression nuts when creating a compression fitting, either between two pieces of pipe or between a pipe and a check valve.
	A compression fitting is used to join two lengths of pipe, usually in small or inconvenient places where pipe welding can be difficult or dangerous. For example, under the sink or behind the toilet where space is limited. The compression nut is tightened around the joint with a wrench, which then seals the olive around the pipe, creating a watertight seal.
Why use a compression fitting?
	The compression fitting is used where welding the connection may be too dangerous or difficult. Compression joints are also used for joints that may need to be disassembled later.

	Benefits of using compression fittings: They are widely available at most DIY stores. They can be reused and replaced (with a new olive) in case of dismantling.
	Disadvantages of using compression fittings: They are often considered clumsy compared to solder joints. Tightening connections can be difficult in awkward places.