The present invention discloses an air delivery system (30) for providing a supply of air from a source of air at positive pressure to a patient, the air delivery system comprising:an interfacing structure (20) located at the entrance to the airways of the patient； anda manifold (70) adapted to connect with the source of air at positive pressure；a pair of tubes (42), each tube being connected to the manifold and adapted to extend along a respective side of the patient's face between a patient's eye and a patient's ear and deliver the supply of air to the interfacing structure,a back strap (60) provided to the tubes and adapted to engage the back of the patient's head；wherein each tube has at least one portion that is structured to allow movement between an open phase in which the tube allows the flow of air without undue resistance and at least a partially collapsed phase in which the tube is at least partially collapsed to substantially impede or prevent the flow of air；wherein each tube has sufficient strength to maintain the open phase without being pressurized.
