The invention relates to a mechanism (2) designed for a chair, in particular for a rotatable office chair having a gas spring (12) vertically arranged in the underframe (1), the gas spring having a telescopically extendible piston rod so that the height of the seat (9) can be adjusted. The tilting adjustment of the seat (9) between the rest position (S0) and the relaxation position (Smax) occurs in synchronization with the adjustment of the backrest (96) between a rest position (R0) and a relaxation position (Rmax) inclined backward. The mechanism (2) has a shell (30), through which a stationary first rotational axis (D1) extends and on which a seat carrier (80) capable of being inclined is mounted. A front-side connection means (33) is arranged between the seat carrier (80) and the shell (30), the front-side connection means being connected to the shell (30) in an articulated manner in a second rotational axis (D2) and being connected to the seat carrier (80) in an articulated manner in a third rotational axis (D3). A rear-side connection means (60) is provided, which is connected in an articulated manner in the first rotational axis (D1) and in a fourth rotational axis (D4), which extends through the seat carrier (80). A spring system (46) acting between the shell (30) and the seat carrier (80) is provided. The mechanism (2) has a force adjuster (39), which can be actuated from the outside and which is designed to change the preloading of the spring system (46) by means of a transmission arrangement (390,51,52,53). The transmission arrangement (390,51,52,53) comprises movable means (52,53) having inclined surfaces provided thereon, wherein a rear-side bearing (462,464,465) of the spring system (46) is supported on said inclined surfaces.