Abstract: A tensioner for the control element of a covering for an architectural opening includes a base member attachable to a support surface adjacent to the architectural opening with the base member supporting an upstanding plunger about which a surrounding housing is adapted to slide. A coil spring biases the housing downwardly relative to the plunger and a rotatable pulley is mounted in an open top of the housing around which an endless operating element for a covering for an architectural opening extends. Operation of the covering by pulling on the endless element raises the housing relative to the plunger permitting free rotation of the pulley. Upon completion of operation the housing lowers under the bias of the coil spring so that the pulley is seated in engagement with the plunger and restrained from rotation.