A patient interface comprising: a seal-forming structure 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 pressurized air at a therapeutic pressure of at least 6 cmH 2 O above ambient air pressure throughout the patient's respiratory cycle in use; a plenum chamber pressurisable to the therapeutic pressure of at least 6 cmH2O above ambient air pressure; and positioning and stabilising structure to provide a force to hold the seal-forming structure in a therapeutically effective position on a patient's head, the positioning and stabilising structure comprising: at least one gas delivery tube coupled to the plenum chamber and configured to receive the flow of pressurized air from a connection port on top of the patient's head and to deliver the flow of pressurized air to the entrance of the patient's airways via the plenum chamber, the at least one gas delivery tube 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, the at least one gas delivery tube comprising a tube wall having an interior passage for flow of pressurized air along a longitudinal axis of the tube to the seal-forming structure, wherein at least a portion of the tube wall comprises: a patient contacting portion comprising a layer of textile material or foam material configured to lie against the patient's head in use; and a non-patient contacting portion, wherein at least a section (3355, 3356) of the non-patient contacting portion is comprised of a transparent and/or translucent material to allow viewing of the interior passage from outside, wherein the layer of textile material or foam material is bonded to the transparent and/or translucent material so that the tube wall is formed as a one piece construction; wherein a plane extending generally transverse to the longitudinal axis contains both the layer textile material or foam material and the transparent and/or translucent material, so that the patient may view the interior passage along a transverse axis extending through the plane; and wherein a portion of the section of transparent and/or translucent material is configured as a rigidising element. a seal-forming structure 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 pressurized air at a therapeutic pressure of at least 6 cmH 2 O above ambient air pressure throughout the patient's respiratory cycle in use; a plenum chamber pressurisable to the therapeutic pressure of at least 6 cmH2O above ambient air pressure; and positioning and stabilising structure to provide a force to hold the seal-forming structure in a therapeutically effective position on a patient's head, the positioning and stabilising structure comprising: at least one gas delivery tube coupled to the plenum chamber and configured to receive the flow of pressurized air from a connection port on top of the patient's head and to deliver the flow of pressurized air to the entrance of the patient's airways via the plenum chamber, the at least one gas delivery tube 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, the at least one gas delivery tube comprising a tube wall having an interior passage for flow of pressurized air along a longitudinal axis of the tube to the seal-forming structure, wherein at least a portion of the tube wall comprises: a patient contacting portion comprising a layer of textile material or foam material configured to lie against the patient's head in use; and a non-patient contacting portion, wherein at least a section (3355, 3356) of the non-patient contacting portion is comprised of a transparent and/or translucent material to allow viewing of the interior passage from outside, wherein the layer of textile material or foam material is bonded to the transparent and/or translucent material so that the tube wall is formed as a one piece construction; wherein a plane extending generally transverse to the longitudinal axis contains both the layer textile material or foam material and the transparent and/or translucent material, so that the patient may view the interior passage along a transverse axis extending through the plane; and a patient contacting portion comprising a layer of textile material or foam material configured to lie against the patient's head in use; and a non-patient contacting portion, wherein at least a section (3355, 3356) of the non-patient contacting portion is comprised of a transparent and/or translucent material to allow viewing of the interior passage from outside, wherein the layer of textile material or foam material is bonded to the transparent and/or translucent material so that the tube wall is formed as a one piece construction; wherein a plane extending generally transverse to the longitudinal axis contains both the layer textile material or foam material and the transparent and/or translucent material, so that the patient may view the interior passage along a transverse axis extending through the plane; and wherein a portion of the section of transparent and/or translucent material is configured as a rigidising element.