A seal-forming structure for a patient interface may include a patient-contacting surface configured to engage the patient's facial skin to form a seal; 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 a support structure extending from the patient contacting surface to an interior surface of the seal-forming structure, the support structure and the interior surface forming a continuous loop, 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.