compression fittings form a seal by applying force. This is done with the three piece assembly; (comprised of a) compression nut, connector and an inner ring (the ferrule). When the compression nut is properly tightened, the ferrule is pressed against the tube with enough force to eliminate excess space in the joint, which prevents fluid or gas from leaking. The ferrule forms the seal, not the nut, the threads or any mechanical connection. This takes all the pressure off the mechanical connection.