A patient interface comprising: a plenum chamber for a patient interface, the plenum chamber being pressurisable to a therapeutic pressure of at least 6 cmH2O above ambient air pressure throughout a patient's respiratory cycle in use, the plenum chamber comprising: one or more walls at least partially enclosing a volume of space; 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 comprising a nasal portion having at least one nasal hole configured to deliver a flow of air at said therapeutic pressure to an entrance to the patient's nares in use, the seal-forming structure comprising an oral portion having an oral hole configured to deliver the flow of air at said therapeutic pressure to an entrance to the patient's mouth in use, the seal-forming structure constructed and arranged to maintain said therapeutic pressure in the plenum chamber throughout the patient's respiratory cycle in use; and one or more plenum chamber inlet ports sized and structured to receive the flow of air at the therapeutic pressure for breathing by the patient throughout the patient's respiratory cycle in use a positioning and stabilising structure configured to generate a force to hold the seal-forming structure in a therapeutically effective position on the patient's head, wherein the seal-forming structure comprises a central portion configured to seal against an inferior periphery of the patient's nose in use, and wherein the seal-forming structure comprises intermediate portions configured to contact the ala of the patient's nose in use, the intermediate portions having a greater stiffness than the central portion; wherein the nasal portion comprises a pair of posterior corners each configured to contact the patient's face in an area lateral and inferior to a respective one of the ala of the patient's nose in use, the posterior corners having a greater thickness than the intermediate portions. a plenum chamber for a patient interface, the plenum chamber being pressurisable to a therapeutic pressure of at least 6 cmH2O above ambient air pressure throughout a patient's respiratory cycle in use, the plenum chamber comprising: one or more walls at least partially enclosing a volume of space; 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 comprising a nasal portion having at least one nasal hole configured to deliver a flow of air at said therapeutic pressure to an entrance to the patient's nares in use, the seal-forming structure comprising an oral portion having an oral hole configured to deliver the flow of air at said therapeutic pressure to an entrance to the patient's mouth in use, the seal-forming structure constructed and arranged to maintain said therapeutic pressure in the plenum chamber throughout the patient's respiratory cycle in use; and one or more plenum chamber inlet ports sized and structured to receive the flow of air at the therapeutic pressure for breathing by the patient throughout the patient's respiratory cycle in use one or more walls at least partially enclosing a volume of space; 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 comprising a nasal portion having at least one nasal hole configured to deliver a flow of air at said therapeutic pressure to an entrance to the patient's nares in use, the seal-forming structure comprising an oral portion having an oral hole configured to deliver the flow of air at said therapeutic pressure to an entrance to the patient's mouth in use, the seal-forming structure constructed and arranged to maintain said therapeutic pressure in the plenum chamber throughout the patient's respiratory cycle in use; and one or more plenum chamber inlet ports sized and structured to receive the flow of air at the therapeutic pressure for breathing by the patient throughout the patient's respiratory cycle in use a positioning and stabilising structure configured to generate a force to hold the seal-forming structure in a therapeutically effective position on the patient's head, wherein the seal-forming structure comprises a central portion configured to seal against an inferior periphery of the patient's nose in use, and wherein the seal-forming structure comprises intermediate portions configured to contact the ala of the patient's nose in use, the intermediate portions having a greater stiffness than the central portion; wherein the nasal portion comprises a pair of posterior corners each configured to contact the patient's face in an area lateral and inferior to a respective one of the ala of the patient's nose in use, the posterior corners having a greater thickness than the intermediate portions.