Abstract: An adjustable register includes a frame member having an air passage therethrough, a plurality of slats connected pivotally to the frame member, a slat connecting member connected to each of the slate a slat-controlling linkage connected pivotally to one of the slats, a toggle-joint mechanism connected pivotally to the slat-controlling linkage on the frame member, and a plurality of blocking bars on the frame member. As the toggle-joint mechanism is operated, the slat-controlling linkage is driven to drive one of the slats. All slats move concurrently between a closed position where the slats contact closely the blocking bars, and an open position where the slats allow air flow through the air passage. In the closed position, the air-tight effect of the register is ensured.