Abstract: A lock for selectively allowing and preventing relative longitudinal displacement of an outer and inner telescoping tube segments. It comprises a tubular cam adapted to plug into and rotate with one end of the inner tube segment. It has associated with it a pair of arcuate cam followers, each with an outer and an inner cylindrical surface where the outer cylindrical surface can be forced by the action of the tubular cam to frictionally engage the inner surface of the outer telescoping tube segment upon rotation of the inner telescoping tube segment.