A chair mechanism (1) has a seat support (11) and a backrest support (50) which can pivot about pins on a fixed support (2). They are interconnected so that their tilting is synchronised. A lock (30) locks the seat support (11) at a certain tilt position with respect to the fixed support (2) and therefore in turn locks the backrest support. The lock (30) has lock members (36, 37) with ridges or teeth which interengage to provide positive mechanical locking. The teeth are configured so that tilt bias which is provided by a bias unit (15) urges the teeth into tighter locking engagement once they are engaged. Therefore, accidental release of the actuator does not cause the lock to become released and therefore jerking actions of parts of the chair are avoided. For releasing of the lock, it is necessary for the user to urge parts of the chair against the tilt bias, thereby causing release surfaces on the teeth to slide with respect to each other to urge the locking member (37) away from the locking member (36). The slopes of the surfaces on the teeth are important as they provide for tighter mechanical engagement under the tilt bias and for disengagement when the user urges a chair part against the tilt bias.