At about the same time the Chinese developed one of the strongest and most seaworthy vessels in existence: the junk, which is still used by the peoples of Southeast Asia. Essentially a large, light flat-bottomed box, the junk lacks three components elsewhere regarded as fundamental to ships: keel, and stem- and sternposts (upright members at bow and stern). Its hull is partitioned off by solid bulkheads running lengthwise and crosswise to divide it into watertight compartments. Such bulkheads, which were not adopted by the West until the 19th century, not only make the ship structurally rigid, but also protect it against sinking. The lack of a keel is compensated for by a heavy steering oar or rudder, mounted on the centerline through a watertight housing in the bottom. The rudder can be raised or lowered. The sails are made up of narrow horizontal linen or matting panels, each secured by its own line or sheet, so that each sail can be quickly spread or closed.