A stackable modular unit for a storage structure for flat objects, comprises two lateral, mutually distanced, wall members (2,2′) to which are attached or with which are integral at least two bottom members (4,8,8′) and at least one top member (12,12′). The bottom and top members define supporting and guiding surfaces for the object (CDC). The lateral walls (2,2′) are provided with cut-outs (16,16′) in their lower portions, the upper edges of the walls and the cut-outs being so configured as to permit, in the stacked state of the units, the nesting of the upper portion of each lateral wall of each unit in the cut-out of the corresponding wall of the next-higher of the stacked units. Any one of the objects (CDC), when introduced into, and slid onto the bottom members of, the nth of the stacked units, mechanically joins the nth unit to the (nth - 1) unit.