Ceiling diffusers and grilles are two of the most common types of air distribution systems used in HVAC systems. Ceiling diffusers are typically round or square fixtures that are mounted in the ceiling and used to direct air into a room. They are often made of metal or plastic and feature adjustable vanes that can be used to control the direction and amount of air flow. Grilles, on the other hand, are typically rectangular fixtures that are mounted in the ceiling and used to provide a more uniform distribution of air throughout a room. They are usually made of metal or plastic and feature a variety of louvers or slats that can be adjusted to control the direction and amount of air flow. Both ceiling diffusers and grilles are important components of an HVAC system, as they are used to regulate the temperature and air quality of a space.