As used with forced air HVAC systems, Louvers, Registers, Grilles and Diffusers are coverings for air passages. Louvers are manufactured assemblies of pitched and overlapping horizontal blades or slats. They control the flow of air while inhibiting other elements from entering in a reverse direction. Louvers may be stationary, manually adjustable or mechanically adjustable. A Grille is a louvered or perforated covering for an opening in an air passage located in the wall, ceiling or floor. The air passage may or may not be connected to a system of ductwork. When a damper assembly is attached to the grille, the entire devise is know as a Register. Diffusers are circular, square or rectangular air distribution outlets which are usually located in the ceiling. They are comprised of deflecting blades which discharge supply air in various directions. Diffusers are designed to mix the conditioned air entering the space with the air already contained in the space.