Abstract: A vertical blind vane tilt control lock for manually locking in a rigid position the vane tilt chain actuating mechanism for vertical blinds to prevent undesired movement of the vertical blind vanes by wind or air passing through the vanes. The control chain locking device is rigidly fastened to a wall surface near the chain actuator so that it can be manually engaged or disengaged as desired. The chain control lock includes one or two locking slots so that both sides of the vane tilt actuating chain are held taut from the vertical blind actuating pulley to prevent movement of the vanes.