A cushion comprises a substantially triangularly shaped frame from which extends a membrane. The membrane is spaced away from the rim of the frame, and its outer surface is of substantially the same shape as the rim. The seal forming portion thus contacts both the surface of the wearer's nose and a portion of the wearer's face. The shape of the seal forming portion is particularly suited to effectively seal the difficult region of the facial contour that is the crease between the sides of the nose and the face.
