On avessel'shull, a rib is a lateral structural member which runs betweengunwales and sprouts from thekeel.[1] They are called "ribs" because they resemble the humanrib. The ship's outer planking and inner sheathing are attached to the ribs. For ships that are too large for a rib to be made out of a single piece of wood, the ribs are made of multiple sections calledfuttocks that arescarfed together.[2] The ancient writersPolybius andPlato held that, since the ribs of a ship were the most important part of the ship's framework, then if the ribs were new, then the ship as a whole was new.[3]Iron was first used in shipbuilding to make the ribs and frames, with wood sheathing attached to them, but this proved inadequate compared to replacing the wood sheathing with iron plates.[4]