Casing is used generously in spaces and is often the most visible moulding in a room. Casings are primarily used to go around a door or window, covering the gap between drywall and the door or window frame. Casings are generally thicker than base mouldings, they bring a room together left to right, top to bottom. And like any framing element, the options are endless, from build-ups of multiple pieces to full wraps.