Casing is the visible trim around an interior door or window opening. It covers the junction between the unit and the surrounding wall, giving the opening a finished border. Casing can have a simple, flat profile or a more decorative shape. It is a finish component, separate from the window or door frame itself.
During design planning, compare casing samples with the existing trim, doors and baseboards. Review the proposed width and profile around a complete opening, especially where wall space is tight or openings are close together. The finishing details matter too: painted and stained trim can give the same profile a different appearance. Record the selected casing in the finish schedule so the intended look is clear.