Tensioning device for the cable pull of a window opener

A tensioning device (14) in combination with a cable pull of a cable-pull window opener, the tensioning device (14) being accommodated in a driver (16) of a window pane, the driver (16) being guided along a guide rail and being fixedly connectable to one end of the cable pull, and the tensioning device (14) comprising a spring-loaded directional locking mechanism which can be connected to the other end (8) of the cable pull, wherein the other end (8)of the cable pull is operatively connected in a spring-loaded manner to an expanding element (4) which is arranged between latching arms (18) of a latching element (2) which is guided displaceably in a holder (1) of the driver (16), and when the window opener is operated, the expanding element engages the latching arms (18) causing the latching arms to expand into a mating toothing arrangement (11) in the holder (1) for the purpose of fixing the other end (8) of the cable pull.