Synchronous chair mechanism (1; 10) for simultaneously changing a seat and a backrest of a chair from a zero position, in which the backrest is tilted minimally relative to the seat, to an end position, in which the backrest is tilted maximally relative to the seat, which synchronous chair mechanism (1; 10) has a spring element (8; 80), a plurality of basic elements, a latching structure (31; 310) formed on one of the basic elements and a sliding coupling piece (6; 60), the basic elements comprising a base (2; 20) connectable to a substructure provided for positioning, a backrest support (3; 30) on which the backrest is mountable, and a seat support (4; 40) designed to accommodate a seat, the spring element (8; 80) having a front end and a rear end, and being in operative connection with at least two of the basic elements, the sliding coupling piece (6; 60) being pivotably mounted on the front end of the spring element (8; 80) or on the rear end of the spring element (8; 80), the sliding coupling piece (6; 60), outside the zero position of the synchronous chair mechanism (1; 10), engaging in the latching structure (31; 310) so that the sliding coupling piece (6; 60) and the latching structure (31; 310) are stationarily interconnected, and the sliding coupling piece (6; 60) being decoupled from the latching structure (31; 310) in the zero position of the synchronous chair mechanism (1; 10) such that the sliding coupling piece (6; 60) is movable relative to the latching structure (31; 310), as a result of which an action of the spring element (8; 80) can be changed, characterized in that the seat support (4; 40) is connected to the backrest support (3; 30) in an articulated manner about a second rotational axis (72; 720). the basic elements comprising a base (2; 20) connectable to a substructure provided for positioning, a backrest support (3; 30) on which the backrest is mountable, and a seat support (4; 40) designed to accommodate a seat, a base (2; 20) connectable to a substructure provided for positioning, a backrest support (3; 30) on which the backrest is mountable, and a seat support (4; 40) designed to accommodate a seat, the spring element (8; 80) having a front end and a rear end, and being in operative connection with at least two of the basic elements, having a front end and a rear end, and being in operative connection with at least two of the basic elements, the sliding coupling piece (6; 60) being pivotably mounted on the front end of the spring element (8; 80) or on the rear end of the spring element (8; 80), the sliding coupling piece (6; 60), outside the zero position of the synchronous chair mechanism (1; 10), engaging in the latching structure (31; 310) so that the sliding coupling piece (6; 60) and the latching structure (31; 310) are stationarily interconnected, and the sliding coupling piece (6; 60) being decoupled from the latching structure (31; 310) in the zero position of the synchronous chair mechanism (1; 10) such that the sliding coupling piece (6; 60) is movable relative to the latching structure (31; 310), as a result of which an action of the spring element (8; 80) can be changed, characterized in that the seat support (4; 40) is connected to the backrest support (3; 30) in an articulated manner about a second rotational axis (72; 720).