These words together are usually used to describe the HVAC grilles (see also: diffuser, HVAC). The “supply” grilles provide (or supply) air to a space, while the “return” grilles allow air to return to the HVAC unit(s) to be conditioned again or exhaust the air to the outside. Sometimes light fixtures (see also: luminaires) are designed to be vehicles for air return into the plenum of a ceiling.