the form of a hollow cylinder in order to receive a fastening means for example a screw by means of which the locking unit 10 is fastened to the backrest 3 during the installation a tensioning element 40 is mounted pivotably on the second bearing bolt 52 for this purpose the tensioning element 40 has a tensioning element hole 44 which in the present case is circular and is penetrated by the second bearing bolt 52 the tensioning element 40 is pretensioned toward the rotary latch 20 by means of a third spring 73 in the locked state when the hook mouth 21 of the rotary latch 20 receives the bolt 12 the tensioning element 40 exerts a closing moment on the rotary latch 20 owing to the pretensioning by the third spring 73 as a securing element for this purpose the tensioning element 40 has a tensioning surface 41 which is curved eccentrically with respect to the second bearing bolt 52 and is in non self locking contact with the functional surface 22 of the rotary latch 20 in the present case the tensioning surface 41 is curved in the shape of an arc of a circle and is of convex design a latching pawl 30 is arranged on the second bearing bolt 52 axially next to the tensioning element 40 and is likewise mounted pivotably on the second bearing bolt 52.