A mouth cushion for a mask system includes a side wall, an undercushion extending away from the side wall, and a membrane provided to substantially surround the undercushion and adapted to form a continuous seal around an exterior of a patient's mouth in use. The side wall includes spaced-apart prong support structures that provide annular recesses adapted to support respective nasal prongs, and at least a portion of the undercushion includes a question-mark or sickle-shape configuration when in cross-section.