Abstract: A trim system for exterior siding includes interlocking trim units that may be installed without nails, screws, or other fasteners being visible. The preferred trim system includes an offset between its two back panels, which offset receives, shields, and conceals a sheet(s) of insulation, and a relief portion which gives the trim system a three-dimensional appearance. Other features of the trim system, such as preferred overlapping or bending tabs and corners, help to keep water out of the trim units and away from the insulation and rear surfaces of the siding.