The rear, reverse, unseen, more remote, or less important part of a structure, tool, or object. 2. The support for a more prominent or visible element. 3. The top or exposed side of a slate, tile, or the like, in contrast to the bed. 4. The ridge or top of a horizontal member or structure like a joist, rafter, or roof. 5. A principal rafter. 6. The extrados or top surface of an arch, often buried in the surrounding masonry.