In bird anatomy, the keel or carina is an extension of the sternum (breastbone) which runs axially along the midline of the sternum and extends outward, perpendicular to the plane of the ribs. The keel provides an anchor to which the supracoracoideus and pectoralis attach, thereby providing adequate leverage for flight. Not all birds have keels; in particular, some flightless birds lack a keel structure. Some flightless birds have a keel, such as the penguin; this is because penguins still require strong wing muscles to power underwater movement.