allocated as a result the backrest carrier is moved under the seat when the backrest is inclined backward and in turn is withdrawn from within the seat when the backrest is moved forward to an upright position in order to achieve an optimum motion sequence of the backrest the curvature of the two curved paths of the seat and the backrest are designed so that the momentary pole around which the backrest turns when being inclined is not constant but on the contrary moves on a section of a spiral curve which preferably opens upward and proceeds in the same direction as the incline motion of the backrest application_number US 89080992 A description the principles of the present invention are particularly useful when incorporated in a dental patient chair illustrated in fig1 the upper chair part is composed of a seat 1 connected to a backrest 2 by a curved brace 25 and the backrest 2 is connected by a headrest carrier 26 to a headrest 3 the seat 1 backrest 2 and headrest 3 are carried by an adjustment mechanism generally indicated at 4 that is supported on a support plane or floor 5 the adjustment mechanism 4 is of such a nature that the upper chair part can be adjusted in height according to arrows 6 the backrest can be inclined according to arrows 7 the overall upper chair part can be longitudinally displaced according to the direction of arrows 8 in a horizontal plane to this end the adjustment mechanism is composed of two articulated arms 9 and 10 which are connected to one another in an articulate.