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 seal-forming structure may be 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 cmH2O with respect to ambient air pressure throughout the patient's respiratory cycle in use. 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 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.