Abstract: An extensible support structure comprising a housing formed of a pair of U-shaped members arranged to form an elongated passageway which telescopically receives a third member characterized by a cam member extending through aligned apertures in the first and second member and slots in the third member, overcenter means received in the housing and coacting with the cam member to urge it into a first position clamping the first and second members onto the third member to frictionally lock the third member in a selected axial position within the housing and a second position releasing the clamping engagement and the frictional lock of the third member.