Abstract: A modular finishing system for framed door or window openings which is adjustable for varying depths of door or window casings. The modular finishing system for framed door or window openings has opposing first and half subassemblies which may be prefabricated. Each subassembly includes casing and trim components. To erect, one subassembly is inserted within the framed door or window opening, one subassembly from each side of the opening. The subassemblies are slideably and adjustably engageable one with the other by a mechanism extending between the first subassembly inner edge and the second subassembly inner edge. In the preferred embodiment of the invention, a cam shim is provided which assists and simplifies squaring the subassemblies within the framed opening.