Abstract: The invention relates to an extendable mast or column assembly comprising a pair of interengaging elongate link chains (13, 14), each link chain comprising a plurality of pivotally interconnected chain link elements (17), the adjacent interconnected link elements (17) of the individual chains (13, 14) having interfitting walls which permit relative pivotal movement from an axially in line position in one direction only, at least some of the link elements (17) having abutment surfaces (23, 24, 34) which, when the chains are interengaged, cooperate with one another to provide interlocking means for preventing relative movement between the link elements of one chain and adjacent link elements of the other chain, and chain drive means (46) for interengaging the chains (13, 14) and driving the chains, so interengaged, in an upwards direction to form the rigid mast or column (10). Guy wires (66) may be employed to ensure that the mast (10) is held secure when in an elevated condition.