Abstract: A positioner for a movable window sash embodies a guide bracket carried by the sash with one side of the guide bracket spaced from and facing an adjacent side of a window frame and extending in a plane which is inclined relative to a plane extending in the direction of movement of the sash with the opposite side of the guide bracket being parallel to said one side thereof. A wedge-like member has a first surface in sliding engagement with said one side of the guide bracket. A second surface of the wedge-like member extends parallel to and faces the adjacent side of a window frame and is movable into and out of engagement with the window frame in response to sliding movement of the wedge-like member in opposite directions. An actuator mounted for sliding movement along said opposite side of the guide bracket is operatively connected to the wedge-like member to move it in opposite directions. A resilient member urges the wedge-like member into engagement with the adjacent side of the window frame.