Abstract: The inner end of a tower crane boom is connected to the top of its tower by two lugs fixed on the tower, each having an upwardly opening slot, and two trunnions fixed on the inner end portion of the boom, one for each lug, each rotatably seatable in the rounded bottom of the slot in its lug. The slot bottom in one lug defines a first fixed axis, midway between the front and rear sides of the tower, about which the boom swings in its range of working positions; that in the other lug defines a second fixed axis, near the front side of the tower, about which the boom swings to and from an inoperative position wherein it extends along the front side of the tower. The distance between the trunnion axes equals the distance between the fixed axes. Links confine the boom against moving to a position in which neither trunnion is seated in the slot in its lug.