A nasal cushion for a mask system includes a side wall defining a breathing chamber, an undercushion extending from the side wall, and a membrane that at least partially covers the undercushion. The membrane 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 is only provided along the side of nose, corner of nose, and upper lip regions of the cushion. The undercushion includes a flap or extending portion in each side of nose region 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.