Abstract: A window casing system includes a kit to completely finish the insides of windows in homes and offices. An installer provides key measurements of the window opening height, width, and depth, to the factory. Custom pre-finished jambs, casings, sills, and aprons are then packaged and delivered in a kit to the jobsite for installation. Plastic fasteners with ribbed shanks are included in the kit for the installer to press into holes and dados in the jambs, casings, and sill to permanently join them together. The top corner miters of the side and top casings are joined with unique, ribbed plastic corner elbows for tight, perfectly aligned joints. The whole is then either nailed, glued, or otherwise fastened into the window opening.