Abstract: A window covering drive system mountable in a sash frame, such as an interior sash, for use in operating a window covering mounted to the sash. The drive system including a drive component couplable to the covering, the drive component including a belt and a belt tensioning mechanism providing pre-tension in the belt and sustaining tension during operation of the drive system. The drive system also including an operator assembly slidably mounted to the sash and coupled to the drive component, such that linear motion of the operator assembly results in movement of the belt and rotary action of the drive component to operate the covering. The drive system may include a frame connecting corner lock configured with features that couple to the drive component. Additionally, a covering operating mechanism may be included having a lift spool with a varying diameter about which a lift cord is wound and unwound.