Exterior Window and Door Trim. Window trim usually more modest than trim for an exterior door, but the two share basic design features. For example, if an entry door features a crosshead pediment than the window trim, at least on the first floor, will often have similar pediments, albeit usually smaller ones. Sometimes, however, the difference between door and window trim treatments is stark. Early designs, such as Georgian and Federal, had very plain window trim compared to the entry door trim, which was often the most elaborate decoration on the whole building. Window casings are easy to install. After the window is placed in the rough wall opening, the casing boards are nailed to the window jambs and to the wall sheathing and studs that define the rough opening. Except for exposure to weather, these boards function the same way as interior casing trim boards. Because the siding abuts these casing boards, the caulk joint between the siding and trim must be well maintained to keep water from penetrating into the building.