A grille is a perforated cover for an air duct (used for heating, cooling, or ventilation, or a combination thereof). Grilles sometimes have louvers which allow the flow of air to be directed. A register is a grille with moving parts, capable of being opened and closed and the air flow directed, which is part of a building’s heating, ventilation, and air conditioning (HVAC) system. The placement and size of registers is critical to HVAC efficiency. Register dampers are also important, and can serve a safety function.