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 patients face surrounding an entrance to the patients 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 patients respiratory cycle in use. The positioning and stabilising structure may comprise a front hoop arranged to contact, in use, at least a region of the patients head superior to an otobasion superior of the patients head and a rear strap. The positioning and stabilising structure may comprise an adjustment mechanism for adjustment of the front hoop and the rear strap relative to the patients 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.