back rest side annular projections 10 2 10 3 which face one another which are provided with at least one claw like axial projection 11 which is arranged in this case on an annular projection 10 3 said annular projection engages in at least one complementary axial recess 12 which in this case is arranged on the opposing annular projection 10 2 so that the annular projections 10 2 10 3 and thus also the back rest segments 5 5 u2032 are restricted in their pivotability relative to one another fig4 a is a view from the front fig4 b is a view from the rear in particular it is thereby prevented that the lateral back rest segment 5 u2032 may be folded onto the seat part 3 when the back rest segment 5 is upright whereby a safety belt would no longer be available to a seat occupant in the region of the central back rest segment 5 in particular in fig4 a and 4 b it is also visible that the projection 11 has a certain play in the recess 12 so that the fittings 9 2 9 3 may also be adjusted independently of one another to a certain extent this is important in particular for adjusting the comfort of the two back rest segments 5 5 u2032 the person skilled in the art recognizes that a plurality of projections 11 and a plurality of recesses 12 may.