a combination key includes a hollow head , a hollow stem connected to the head and a plurality of teeth mounted on racks longitudinally movable in the head and stem . the racks are in meshed engagement with toothed control wheels which are mounted in the head and emerge partially to the outside . the control wheels are located in the head next adjacent the stem . the length of the head is at least equal to the length of the stem so that the racks can be accommodated in the head when the teeth are in a position of maximum retraction .