Disclosed is an air terminal device for a ventilation system for a building. The air terminal device includes a pressure box, with an inlet admitting supply air into the pressure box and outlet openings for admitting supply air out. The device further includes first and second cover plates, each arranged to control and change the open area of a plurality of the outlet openings. The outlet openings are arranged in a wall of the pressure box forming an outlet surface of the pressure box. The cover plate or cover plates are arranged to be in contact with and slide relative the outlet surface while changing the open area of the outlet openings. The first cover plate and the second cover plate are arranged to slide relative each other and to be in contact with and slide relative the outlet surface while changing the open area of the outlet openings.