The hull is built around the keel. External keels are cast in the shape of the keel. The longship's narrow deep keel provided strength beneath the waterline. A centerboard is mounted in lieu of a keel. The caudal fin has a strong lateral keel and a crescent shape. She rolled badly until fitted with a bilge keel. The design included an even keel and nearly horizontal propeller shafts to minimize weight. Young map turtles have a pronounced dorsal keel. Both muscle groups attach to the keel of the sternum. Spring in the keel or rocker influences how a rowboat performs. The sternum is small and has a low, vestigial keel and a shortened spina externa. Each osteoderm possesses a pronounced central keel and an anterior projection. By 1236, the cruiser was back on an even keel. Its characteristics were a long cranium with a sagittal keel and heavy browridge. Modern monocoque designs include a virtual keel.