A cushion member for a patient interface for delivery of a supply of pressurised air or breathable gas to an entrance of a patient's airways, the cushion member comprising: a frame member; a retaining structure for repeatable engagement with and disengagement from the frame member; and a seal-forming structure having a nasal opening to provide pressurized gas to both nares of the patient and a plenum chamber formed in one piece, the seal-forming structure configured to seal around an inferior periphery of the patient's nose and below the bridge of the nose, and the seal-forming structure and plenum chamber permanently connected to the retaining structure; wherein the seal-forming structure is made from a first material and the retaining structure is made from a second material with different mechanical characteristics from the first material and the second material is more rigid than the first material; and wherein the plenum chamber comprises a sealing lip configured to form a pneumatic seal between the plenum chamber and the frame member when the plenum chamber is secured to the frame member by interference from the sealing lip deflecting against the frame member, the sealing lip being constructed from a flexible resilient material, the sealing lip being located on or formed as part of an interior surface or interior periphery of the plenum chamber, or an entire interior peripheral region of the plenum chamber, characterised in that: the sealing lip is arranged to be urged against the frame member with greater force by an increase in air pressure within the cushion member to cause a sealing force between the seal-forming structure and the frame member to increase. a frame member; a retaining structure for repeatable engagement with and disengagement from the frame member; and a seal-forming structure having a nasal opening to provide pressurized gas to both nares of the patient and a plenum chamber formed in one piece, the seal-forming structure configured to seal around an inferior periphery of the patient's nose and below the bridge of the nose, and the seal-forming structure and plenum chamber permanently connected to the retaining structure; wherein the seal-forming structure is made from a first material and the retaining structure is made from a second material with different mechanical characteristics from the first material and the second material is more rigid than the first material; and wherein the plenum chamber comprises a sealing lip configured to form a pneumatic seal between the plenum chamber and the frame member when the plenum chamber is secured to the frame member by interference from the sealing lip deflecting against the frame member, the sealing lip being constructed from a flexible resilient material, the sealing lip being located on or formed as part of an interior surface or interior periphery of the plenum chamber, or an entire interior peripheral region of the plenum chamber, characterised in that: the sealing lip is arranged to be urged against the frame member with greater force by an increase in air pressure within the cushion member to cause a sealing force between the seal-forming structure and the frame member to increase.