Add character to your home. From the inside, a custom Bay window adds warmth and charm to your family room, kitchen or living room. Display plants and collectibles on the seat, or just curl up with a good book in the sunlight. Bay window always features three projected and joined window units, the two side units are usually vented and the middle is fixed. The head and seat boards of a Bay windows are finished in either oak or birch 1.25" thick externally grated plywood.