A nasal seal for a mask interface, the seal comprising: • a face contacting or wearer side comprising a supple lower-nose-receiving center part having a concave shape preformed to receive and sealingly contact the tip, lower sides, and base of the nose of a wearer, to sealingly contact the upper lip, and to position an aperture for gas flow beneath the nares of the wearer, the supple lower-nose-receiving center part comprising left and right side wall portions (43, 44) on left and right sides of the aperture to contact the left and right lower sides of the nose of the wearer in use; and • an outward side of the seal opposite the face contacting or wearer side of the seal, wherein: • the seal comprises a hollow interior at least on an internal side of the supple lower-nose-receiving center part of the wearer side of the seal, the hollow interior configured to be filled with gas under positive pressure in use; the outward side of the seal comprising a central opening for gas flow into and from the hollow interior of the seal; the seal comprising a rim around said opening for attaching or connecting the seal to a mask frame; the hollow interior being in fluid communication with both the opening and the aperture and providing a gas flow path between the opening and the aperture; • the left and right side wall portions (43,44) or at least upper sections of the left and right side wall portions (43,44) of the face contacting or wearer side have an outwardly projecting or convex shape on opposite sides of the seal, and • the outward side of the seal comprises left and right outward-side stabilising outer parts (63,64) which terminate in left and right outer tips (67,68) which contact the face adjacent the lower flanks of the nose on the left and right sides of the nose when worn, said left and right outward-side stabilising outer parts (63,64) of the seal being less flexible relative to, or stiff relative to, at least the lower-nose-receiving center part of the wearer side of the seal; and the wearer side of the seal comprises left and right wearer-side outer stabilising parts (53, 54) outward of the left and right side wall portions of the lower-noise-receiving center part, to contact the wearer's face adjacent sides of left and right sides of the nose, and the left and right wearer-side outer stabilising parts being flexible but less so than the lower-nose-receiving center part of the wearer side of the seal, said left and right wearer-side stabilising outer parts having a thicker wall section than a wall section of the lower-nose-receiving center part of the wearer side of the seal. • a face contacting or wearer side comprising a supple lower-nose-receiving center part having a concave shape preformed to receive and sealingly contact the tip, lower sides, and base of the nose of a wearer, to sealingly contact the upper lip, and to position an aperture for gas flow beneath the nares of the wearer, the supple lower-nose-receiving center part comprising left and right side wall portions (43, 44) on left and right sides of the aperture to contact the left and right lower sides of the nose of the wearer in use; and a supple lower-nose-receiving center part having a concave shape preformed to receive and sealingly contact the tip, lower sides, and base of the nose of a wearer, to sealingly contact the upper lip, and to position an aperture for gas flow beneath the nares of the wearer, the supple lower-nose-receiving center part comprising left and right side wall portions (43, 44) on left and right sides of the aperture to contact the left and right lower sides of the nose of the wearer in use; and • an outward side of the seal opposite the face contacting or wearer side of the seal, wherein: • the seal comprises a hollow interior at least on an internal side of the supple lower-nose-receiving center part of the wearer side of the seal, the hollow interior configured to be filled with gas under positive pressure in use; the outward side of the seal comprising a central opening for gas flow into and from the hollow interior of the seal; the seal comprising a rim around said opening for attaching or connecting the seal to a mask frame; the hollow interior being in fluid communication with both the opening and the aperture and providing a gas flow path between the opening and the aperture; • the left and right side wall portions (43,44) or at least upper sections of the left and right side wall portions (43,44) of the face contacting or wearer side have an outwardly projecting or convex shape on opposite sides of the seal, and • the outward side of the seal comprises left and right outward-side stabilising outer parts (63,64) which terminate in left and right outer tips (67,68) which contact the face adjacent the lower flanks of the nose on the left and right sides of the nose when worn, said left and right outward-side stabilising outer parts (63,64) of the seal being less flexible relative to, or stiff relative to, at least the lower-nose-receiving center part of the wearer side of the seal; and