Abstract: A chair control mechanism for a chair assembly having a seat and a back includes an arrangement for mounting the lower end of a back support member, for positioning the back above the seat. The chair control mechanism includes a variable configuration mounting area so as to enable different back support members to be used with the same basic chair control mechanism structure. In one version, the variable configuration mounting area is provided by engaging one of a series of inserts within a transverse rear wall defined by the chair control mechanism. Each insert defines a passage configured to receive the lower seat mounting section of a back mounting member having a different set of dimensions, and the seat mounting section of the back mounting member extends through the passage and is engaged with the chair control mechanism by means of a fixed fastener-type arrangement or by a hand screw received within a slot for providing adjustability in the position of the back relative to the seat.