A positioning and stabilising structure configured 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: two gas delivery tubes, each of the two gas delivery tubes being configured to deliver the flow of air to the entrance of a patient's airways via the seal-forming structure, each of the two gas delivery tubes being constructed and arranged to contact, in use, at least a region of the patient's head superior to an otobasion superior of the patient's head; a bias and adjustment mechanism configured to allow adjustment of a length of each of the two gas delivery tubes to enable the positioning and stabilising structure to fit different size heads and configured to impart a biasing force along at least a part of a length of each of the two gas delivery tubes to urge the seal-forming structure towards the entrance of the patient's airways in use; a rear strap connected between the two gas delivery tubes and configured, in use, to pass around the posterior portion of the patient's head; and a connection port configured to fluidly connect, in use, with an air circuit connected to a supply of pressurised air, the connection port configured to be located, in use, superior to the patient's head, wherein each of the two gas delivery tubes comprises a first end configured to be connected to the seal-forming structure, wherein the bias and adjustment mechanism comprises an elastic member provided between the seal-forming structure and the connection port, wherein the elastic member comprises a portion of the gas delivery tube having a concertina structure, wherein the portion of the gas delivery tube having the concertina structure is, in use, positioned in contact with a region of the patient's head superior to the patient's otobasion superior, and the positioning and stabilizing structure being characterized in that: the rear strap is connected to each of the two gas delivery tubes between the portion of the gas delivery tube having the concertina structure and the first end. two gas delivery tubes, each of the two gas delivery tubes being configured to deliver the flow of air to the entrance of a patient's airways via the seal-forming structure, each of the two gas delivery tubes being constructed and arranged to contact, in use, at least a region of the patient's head superior to an otobasion superior of the patient's head; a bias and adjustment mechanism configured to allow adjustment of a length of each of the two gas delivery tubes to enable the positioning and stabilising structure to fit different size heads and configured to impart a biasing force along at least a part of a length of each of the two gas delivery tubes to urge the seal-forming structure towards the entrance of the patient's airways in use; a rear strap connected between the two gas delivery tubes and configured, in use, to pass around the posterior portion of the patient's head; and a connection port configured to fluidly connect, in use, with an air circuit connected to a supply of pressurised air, the connection port configured to be located, in use, superior to the patient's head, wherein each of the two gas delivery tubes comprises a first end configured to be connected to the seal-forming structure, wherein the bias and adjustment mechanism comprises an elastic member provided between the seal-forming structure and the connection port, wherein the elastic member comprises a portion of the gas delivery tube having a concertina structure, wherein the portion of the gas delivery tube having the concertina structure is, in use, positioned in contact with a region of the patient's head superior to the patient's otobasion superior, and the positioning and stabilizing structure being characterized in that: the rear strap is connected to each of the two gas delivery tubes between the portion of the gas delivery tube having the concertina structure and the first end.