Casing – Casing in the broadest sense of the word refers to any outer covering. In the world of home design and construction, casing – is the frame or framework of a window or a door. Anyone building a house can tell you that doors and windows can be a big expense. Well-built casing allows those doors and windows to perform and operate correctly. Sometimes a casing doesn’t even need a door or window to go inside. This would be called a cased-opening.