A patient interface for sealed delivery of a flow of air at a continuously positive pressure with respect to ambient air pressure to an entrance to the patient's airways including at least an entrance of a patient's nares, wherein the patient interface is configured to maintain a therapy pressure in a range of about 4 cmH 2 O to about 30 cmH 2 O above ambient air pressure in use, throughout the patient's respiratory cycle, while the patient is sleeping, to ameliorate sleep disordered breathing, said patient interface comprising: a seal forming structure to form a seal with the entrance to the patient's airways including at least the entrance of the patient's nares; a plenum chamber pressurised at a pressure above ambient pressure in use, the seal forming structure attached to the plenum chamber; and a positioning and stabilising structure to maintain the seal forming structure in sealing contact with an area surrounding the entrance to the patient's airways while maintaining a therapeutic pressure at the entrance to the patient's airways, wherein the positioning and stabilising structure is connected to the plenum chamber such that forces imposed on the positioning and stabilising structure by movement of the patient's head are decoupled from the plenum chamber and the seal forming structure, wherein the positioning and stabilising structure comprises a rigidiser arm assembly, the rigidiser arm assembly being flexibly attached to the plenum chamber, wherein the rigidiser arm assembly is flexibly attached to a medial, anterior surface of the plenum chamber, the patient interface being characterized in that the rigidiser arm assembly comprises two rigidiser arms, each of the rigidiser arms configured to pass along one of the patient's cheeks in use, and wherein the plurality of straps comprises two side straps, each of the side straps configured to connect to one of the rigidiser arms, and each of the side straps configured to pass below the patient's eye and above the patient's ear in use. a seal forming structure to form a seal with the entrance to the patient's airways including at least the entrance of the patient's nares; a plenum chamber pressurised at a pressure above ambient pressure in use, the seal forming structure attached to the plenum chamber; and a positioning and stabilising structure to maintain the seal forming structure in sealing contact with an area surrounding the entrance to the patient's airways while maintaining a therapeutic pressure at the entrance to the patient's airways, wherein the positioning and stabilising structure is connected to the plenum chamber such that forces imposed on the positioning and stabilising structure by movement of the patient's head are decoupled from the plenum chamber and the seal forming structure, wherein the positioning and stabilising structure comprises a rigidiser arm assembly, the rigidiser arm assembly being flexibly attached to the plenum chamber, wherein the rigidiser arm assembly is flexibly attached to a medial, anterior surface of the plenum chamber, the patient interface being characterized in that the rigidiser arm assembly comprises two rigidiser arms, each of the rigidiser arms configured to pass along one of the patient's cheeks in use, and wherein the plurality of straps comprises two side straps, each of the side straps configured to connect to one of the rigidiser arms, and each of the side straps configured to pass below the patient's eye and above the patient's ear in use.