Compression fittings (in some cases referred to as "lock-bush fittings") include a tapered, concave conical seat; a hollow, barrel-formed compression ring (in some cases called a ferrule); in addition to a compression nut which is threaded on to your body of the fitting and tightened to generate a leakproof relationship. http://manifoldvalve31841.free-blogz.com/17798879/pipe-fitting-can-be-fun-for-anyone