The hull is built around the keel.[17] Resembling ribs, and fastened at right-angles to the keel are the ship's frames.[17] The ship's main deck, the metal platework that covers the top of the hull framework, is supported by beams that are attached to the tops of the frames and run the full breadth of the ship.[17] The beams not only support the deck, but along with the deck, frames, and transverse bulkheads, strengthen and reinforce the shell.[17] Another feature of recent hulls is a set of double-bottom tanks, which provide a second watertight shell that runs most of the length of a ship.[18] The double-bottoms generally hold liquids such as fuel oil, ballast water or fresh water.[18]