The inventions concerns a mask system for treating sleep disordered breathing, comprising headgear (20), a shell/cushion (30) including a channel (524) adjacent a front aperture (30.3), a frame (40), an elbow (50) including at least one undercut (50.2) on a proximal end, a retaining ring (60) including a rear flange (64) adapted to be retainably insertable in the channel (524) of the shell/cushion (30), and a front flange (66) adapted to retainably engage with the at least one undercut (50.2) of the elbow (50).
