A plenum chamber for a patient interface, the plenum chamber pressurisable to a therapeutic pressure of at least 6 cmH2O above ambient air pressure, the plenum chamber having an oral portion and a nasal portion, the plenum chamber 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, said seal-forming structure having one or more holes therein such that the flow of air at said therapeutic pressure is delivered to an entrance to the patient's nares and to the patient's mouth via the one or more holes, 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 shell having one or more plenum chamber inlet ports sized and structured to receive a flow of air at the therapeutic pressure for breathing by a patient, the shell supporting the seal-forming structure, wherein the shell is provided to the oral portion of the plenum chamber, and substantially the entire nasal portion is formed by the seal-forming structure; and wherein the seal-forming structure comprises lateral support portions located on laterally-spaced at least partially anterior-facing sides of the nasal portion, each lateral support portion provided at an inferior region of a lateral side of the nasal portion, the lateral support portions each having a higher resistance to deformation in use in comparison to an anterior-facing central portion of the seal-forming structure between the lateral support portions, wherein the seal-forming structure comprises lateral-facing posterior portions provided between respective intermediate portions (3121, 3125) and the lateral support portions on each side of the nasal portion, the seal-forming structure having a greater thickness in the lateral-facing posterior portions than in the intermediate portions (3121, 3125). 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, said seal-forming structure having one or more holes therein such that the flow of air at said therapeutic pressure is delivered to an entrance to the patient's nares and to the patient's mouth via the one or more holes, 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 shell having one or more plenum chamber inlet ports sized and structured to receive a flow of air at the therapeutic pressure for breathing by a patient, the shell supporting the seal-forming structure, wherein the shell is provided to the oral portion of the plenum chamber, and substantially the entire nasal portion is formed by the seal-forming structure; and wherein the seal-forming structure comprises lateral support portions located on laterally-spaced at least partially anterior-facing sides of the nasal portion, each lateral support portion provided at an inferior region of a lateral side of the nasal portion, the lateral support portions each having a higher resistance to deformation in use in comparison to an anterior-facing central portion of the seal-forming structure between the lateral support portions, wherein the seal-forming structure comprises lateral-facing posterior portions provided between respective intermediate portions (3121, 3125) and the lateral support portions on each side of the nasal portion, the seal-forming structure having a greater thickness in the lateral-facing posterior portions than in the intermediate portions (3121, 3125).