Abstract: Segments can be telescoped out of the base boom with boom guides of a telescopic boom of a crane, which in either their extended or retracted positions are locked by locking pins. After releasing the locking pin connections, the segments can be moved in and out with a single-stage hydraulic piston-cylinder unit, one end of which is attached to the boom guide through two strokes so that each segment to be moved in or out is coupled either to the inside or the outside end area of the movable part of the piston-cylinder unit. In order to obtain a simpler structure, a movable ring or a movable pipe with a coupling device to couple the segments is placed on the cylinder by the piston rod fastened to the boom guide piece, which can be locked to either the internal or the external end area of the cylinder.