Window casing. The traditional method of casing a window (or door) is to use separate side, top and bottom pieces. Because each piece is distinct, the trim can be richly detailed and articulated. For example, along the top of the windows there can be a horizontal band that provides a base for a large, flat band that holds a shaped crown. This is a semicustom approach, as each piece can be purchased, or milled, and installed separately, getting you the exact profile and look you'll want.