For Copper Pipes
Copper pp compression fitting has relatively simple structure, it is composed of just 3 parts: a compression seat with the external threads, a brass ring of full circle and a compression nut.

For Pex Pipes
Pex compression fittings always have a outlet nipple either one side or all sides for water to go through and there is always a gap in the brass ring that enables the ring to squeeze flexibly while tighterning the put with the nuts.

For Pex-al-pex Pipes
The only difference between Pex-al-pex compression fitting and pex compression fitting is that there are two more rubber O-rings on each outlet nipple.