In known foldable chair frame constructions, such as in the lie-back push chair disclosed in European Patent Specification No. 0 000 937, the seat back is supported in its upright and lie-back positions by adjustable links extending respectively from rear legs of the frame and back rest members respectively disposed on opposite sides of the seat back. In order to move the seat back between its upright and lie-back positions, both links must be released, moved to their new supporting positions, and then relocked in position. Moreover, during this operation, it is normally preferable to ensure that the chair frame is not supporting any load. Thus, in the case of a push chair, it is necessary to remove a child carried in the push chair whenever the seat back is moved between its upright and lie-back positions.