A nasal cushion (30) comprises a substantially triangularly shaped frame (32) from which extends a membrane (34). The frame (32) has a scalloped edge (36) by which the cushion (30) is affixed to a mask body. The membrane (34) has an aperture (38) into which the wearer's nose is received. The membrane (34) is spaced away from the rim (40) of the frame (32), and its outer surface (41) is of substantially the same shape as the rim (40). Respective notches (42,44) receive the bridge of the wearer's nose. The wearer's nose is received through the aperture (38) into the chamber within the mask body (46). The seal forming portion (45) thus contacts both the surface of the wearer's nose and a portion of the wearer's face in the region between the base of the nose and the upper lip, and around the sides and over the bridge of the nose. The shape of the seal forming portion (45) 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.