A tensioning device for tensioning a flexible elongate member, such as a cord, comprises a first winding member which can rotate around an axle for, directly or indirectly, winding and unwinding the elongate member at a first end thereof, and a second winding member which can rotate around said axle for, directly or indirectly, winding and unwinding the elongate member at the second end thereof. The first and second winding members are interconnected by a tensioning member striving to cause a relative rotation between the first and second winding members for creating a prestress or tension in the elongate member. A restriction device is provided for limiting the maximum possible relative rotation between the first and second winding members.