Sizing Information: The ComfortGel BLUE Nasal mask uses three different mask frame sizes -- Petite, Small/Medium, and Large. Petite cushions and flaps will only fit the Petite frame. Large cushions and flaps will only fit the Large frame. Small and Medium cushions and flaps are interchangeable and can be used on any Small/Medium frame. For proper operation the Comfort Flap must be the same size as the Nasal Cushion it will be used with.