an air flow control register includes a frame carrying pivotable vanes , which are rotated on parallel axes by a common drive mechanism , so that the setting of the vanes is variable between a maximum flow position , in which the flaps have a minimum inclination with respect to the direction of air flow through the register , and a no - flow position in which this inclination is a maximum . the drive mechanism controls the pivoting of the vanes in such a way that they all rotate by the same amount , with alternate vanes rotating in opposite directions .