A chair, in particular an office chair, has a so-called synchronous mechanism in the form of a front seat support member stationary in relation to a chair column and a rear seat support member pivotable about a pivot axle in relation to the front seat support member and bearing a back-rest. A seat supports itself on the two seat support members. A blocking lever of a blocking mechanism and having a stop face is articulated on the rear seat support member at a distance from the pivot axle. This stop face is moved by gravity into engagement with a blocking face stationary in relation to the front seat support member. For releasing this locking, the blocking lever is lifted by a blocking actuator arm.