a frame module;

a cushion module provided to the frame module and adapted to form a seal with the patient's nose and mouth;

an elbow module provided to the frame module and adapted to be connected to an air delivery tube to deliver breathable gas to the patient; and

headgear removably attachable to the frame module, wherein:

the frame module is structured to assist in maintaining the cushion module and the elbow module in an operative position with respect to the patient's face,

the frame module includes an open construction, said frame module being structured to retain the elbow module,

the frame module includes two headgear connectors to connect with the headgear, the two headgear connectors each including a slot at a free end of the connector adapted to receive a respective headgear strap of the headgear,

each said headgear connector is removably coupled to the frame module, each said headgear connector including a clip structure adapted to removably engage with a corresponding portion of the frame module,

the cushion module is structured to interface with the frame module,

the cushion module includes a main body of relatively harder material and a cushion of relatively softer material provided to the main body, the main body defining a breathing chamber and being adapted to interface with the frame module,

the cushion provides a sealing portion adapted to form the seal with the patient's nose and mouth,

wherein a portion of the main body is exposed and remains uncovered by the frame module when the frame module and the main body are engaged,

the main body includes an entry opening for the introduction of the supply of gas into the breathing chamber, the opening being adapted to fluidly communicate with the elbow module,

the cushion is a full-face cushion adapted to engage the patient's face generally along nasal bridge, cheek, and chin regions of the patient's face,

the cushion includes a base wall provided to the main body, an undercushion layer extending away from the base wall, and a membrane provided to substantially cover the undercushion layer,

the undercushion layer is provided in a nasal bridge region of the cushion,

the undercushion layer is not provided in at least a part of a chin region of the cushion,

the frame module is structured to hold the cushion module in an operative position with respect to the patient's face,

the elbow module includes an elbow, a vent arrangement provided to the elbow for gas washout, and an anti-asphyxia valve provided to the elbow,

the elbow includes a first end portion and a second end portion, the first end portion being attached to the frame module, the second end portion being provided to a swivel joint adapted to be connected to the air delivery tube,

the vent arrangement comprises a vent insert that is adapted to be supported within an outlet opening in the elbow, the vent insert having one or more vent holes,

the elbow includes a port that is selectively closed by a flap portion of the anti-asphyxia valve depending on the presence of pressurized gas, and a structure to attach the anti-asphyxia valve, the structure being positioned between the swivel joint and the vent arrangement,

the frame module is structured to maintain the elbow module in an operative position with respect to the patient's face, and

the elbow module is rotatably attached to the frame module so that the elbow module is permitted to be rotated 360 degrees relative to the frame module.