Aspects of the present technology comprise a positioning and stabilising structure to hold a seal-forming structure in a therapeutically effective position on a head of a patient. The positioning and stabilising structure may comprise at least one gas delivery tube to deliver the flow of air to the entrance of a patient's airways via the seal-forming structure. The at least one gas delivery tube may be 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. The positioning and stabilising structure may comprise an adjustment mechanism for adjustment of a length of the at least one gas delivery tube to enable the positioning and stabilising structure to fit different size heads. The positioning and stabilising structure may comprise a bias mechanism to impart a biasing force along at least a part of a length of the at least one gas delivery tube to urge the seal-forming structure towards the entrance of the patient's airways in use.