A patient interface comprising: a plenum chamber pressurisable to a therapeutic pressure of at least 6 cmH 2 O above ambient air pressure, said plenum chamber including a plenum chamber inlet port sized and structured to receive a flow of air at the therapeutic pressure for breathing by a patient, 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, the seal-forming structure constructed and arranged to maintain said therapeutic pressure in the plenum chamber throughout the patient's respiratory cycle in use; a positioning and stabilising structure to provide a force to hold the seal-forming structure in a therapeutically effective position on the patient's head, the positioning and stabilising structure comprising a tie, the tie being constructed and arranged so that at least a portion overlies a region of the patient's head superior to an otobasion superior of the patient's head in use; and a vent structure to allow a continuous flow of gases exhaled by the patient from an interior of the plenum chamber to ambient, said vent structure being sized and shaped to maintain the therapeutic pressure in the plenum chamber in use, wherein the patient interface is configured to allow the patient to breath from ambient through their mouth in the absence of a flow of pressurised air through the plenum chamber inlet port, or the patient interface is configured to leave the patient's mouth uncovered, wherein the seal-forming structure further comprises a patient-contacting surface configured to engage the patient's facial skin to form a seal and a posterior opening formed in the patient-contacting surface, the posterior opening configured to provide the flow of air at said therapeutic pressure to the patient's nares, and wherein the seal-forming structure includes a support structure extending from a first location on the patient contacting surface to a second location on an interior surface of the seal-forming structure, the support structure and the interior surface forming a continuous loop, and wherein a superior portion of the seal-forming structure is structured to engage the patient's nose inferior to the patient's nasal bone. a plenum chamber pressurisable to a therapeutic pressure of at least 6 cmH 2 O above ambient air pressure, said plenum chamber including a plenum chamber inlet port sized and structured to receive a flow of air at the therapeutic pressure for breathing by a patient, 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, the seal-forming structure constructed and arranged to maintain said therapeutic pressure in the plenum chamber throughout the patient's respiratory cycle in use; a positioning and stabilising structure to provide a force to hold the seal-forming structure in a therapeutically effective position on the patient's head, the positioning and stabilising structure comprising a tie, the tie being constructed and arranged so that at least a portion overlies a region of the patient's head superior to an otobasion superior of the patient's head in use; and a vent structure to allow a continuous flow of gases exhaled by the patient from an interior of the plenum chamber to ambient, said vent structure being sized and shaped to maintain the therapeutic pressure in the plenum chamber in use, wherein the patient interface is configured to allow the patient to breath from ambient through their mouth in the absence of a flow of pressurised air through the plenum chamber inlet port, or the patient interface is configured to leave the patient's mouth uncovered, wherein the seal-forming structure further comprises a patient-contacting surface configured to engage the patient's facial skin to form a seal and a posterior opening formed in the patient-contacting surface, the posterior opening configured to provide the flow of air at said therapeutic pressure to the patient's nares, and wherein the seal-forming structure includes a support structure extending from a first location on the patient contacting surface to a second location on an interior surface of the seal-forming structure, the support structure and the interior surface forming a continuous loop, and wherein a superior portion of the seal-forming structure is structured to engage the patient's nose inferior to the patient's nasal bone.