A nasal cushion (6540) for a mask system comprises a side wall defining a breathing chamber, an undercushion (6544) extending from the side wall, and a membrane (6546) that at least partially covers the undercushion (6544). The membrane (6546) is adapted to seal along the nasal bridge, sides of nose, corners of nose, and upper lip of the patient's face in use. The undercushion (6544) is only provided along the side of nose, corner of nose, and upper lip regions (SN, CN, UL) of the cushion (6540). The undercushion (6544) includes a flap or extending portion (6545) in each side of nose region (SN) that is wider than the other regions thereof and adapted to engage and provide a force into the sides of the patient's nose in use.
