A positioning and stabilising structure to hold a seal-forming structure in a therapeutically effective position on a head of a patient, the seal-forming structure being constructed and arranged to form a seal with a region of the patient's face surrounding an entrance to the patient's airways for sealed delivery of a flow of air at a therapeutic pressure of at least 4 cmH 2 O with respect to ambient air pressure throughout the patient's respiratory cycle in use, the positioning and stabilising structure comprising: a front hoop extending, in use, across the patient's cheek regions and arranged to contact, in use, regions of the patient's head superior to an otobasion superior of the patient's head; a rear strap configured, in use, to pass around the back of the patient's head; two gas delivery tubes to deliver the flow of air to the entrance of the patient's airway via the seal-forming structure, wherein the front hoop includes lower portions of each of the two gas delivery tubes, the lower portions of each gas delivery tube extending, in use, across one of the patient's cheek regions, the two gas delivery tubes being on different sides of the patient's head, and a de-coupling mechanism (7600, 8600, 9600) for decoupling positional adjustment of upper portions of the two gas delivery tubes from movement of the seal forming structure away from the patient's face in use to enable positional adjustment of the upper portions of the gas delivery tubes on the patient's head, the two gas delivery tubes fluidly being connected between the de-coupling structure and the seal-forming structure, wherein the de-coupling mechanism is fluidly connected, in use, with an air circuit connected to a supply of pressurised air, the de-coupling mechanism being located, in use, proximal a top, side, or rear portion of a patient's head and comprises the upper portions (7410, 8410, 9411) being bendable and including corrugations and/or concertinas to enable positional adjustment of the upper portions of the gas delivery tubes on the patient's head in use, and wherein the de-coupling mechanism comprises at least one swivel (7408, 8408, 9408) including a y-shaped connection port configured to connect to the air circuit, the at least one swivel (7408, 8408, 9408) configured to allow relative rotation between the upper portions of the two gas delivery tubes and the air circuit connected to the connection port, characterized in that the positioning and stabilising structure comprises: an adjustment mechanism for adjustment of the front hoop and the rear strap relative to the patient's head, the adjustment mechanism being arranged in a single operation to adjust both the front hoop and rear strap to enable the positioning and stabilising structure to fit different size heads. a front hoop extending, in use, across the patient's cheek regions and arranged to contact, in use, regions of the patient's head superior to an otobasion superior of the patient's head; a rear strap configured, in use, to pass around the back of the patient's head; two gas delivery tubes to deliver the flow of air to the entrance of the patient's airway via the seal-forming structure, wherein the front hoop includes lower portions of each of the two gas delivery tubes, the lower portions of each gas delivery tube extending, in use, across one of the patient's cheek regions, the two gas delivery tubes being on different sides of the patient's head, and a de-coupling mechanism (7600, 8600, 9600) for decoupling positional adjustment of upper portions of the two gas delivery tubes from movement of the seal forming structure away from the patient's face in use to enable positional adjustment of the upper portions of the gas delivery tubes on the patient's head, the two gas delivery tubes fluidly being connected between the de-coupling structure and the seal-forming structure, wherein the de-coupling mechanism is fluidly connected, in use, with an air circuit connected to a supply of pressurised air, the de-coupling mechanism being located, in use, proximal a top, side, or rear portion of a patient's head and comprises the upper portions (7410, 8410, 9411) being bendable and including corrugations and/or concertinas to enable positional adjustment of the upper portions of the gas delivery tubes on the patient's head in use, and wherein the de-coupling mechanism comprises at least one swivel (7408, 8408, 9408) including a y-shaped connection port configured to connect to the air circuit, the at least one swivel (7408, 8408, 9408) configured to allow relative rotation between the upper portions of the two gas delivery tubes and the air circuit connected to the connection port,