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An interlock unit (10) for use with at least one other like unit (11) to form an interlock mechanism has a top face (13) and opposed pairs of side walls (12) with at least one runner bar (17, 18) mounted in the unit for lateral sliding movement. An actuator (14, 15, 22) is supported on the top face (13), to effect lateral sliding movement of the runner bar (17, 18). One wall of the unit is provided with one part (27) of a two-part connector with the other part (33) on the opposed wall whereby two similar units may be joined together by relative sliding movement of the two units in the plane of the side walls (12). Joining together of two units (10, 11) in this way also joins together the runner bars (17) or (18) of the two units such that sliding motion of the runner bar (17) of one unit (10) is transferred to the runner bar (17) of the connected unit (11), for at least one direction of sliding movement of the runner bars (17).