The sternal angle, also known as the Angle of Louis, is a protrusion at the joint between the manubrium and the main, lower portion of the sternum. The manubrium is a bone directly above the sternum to which the first and second ribs are connected. The sternal angle is detectable externally by touch and is a useful superficial marker for the location of the second ribs and other anatomical features.
The sternal angle is mostly notable for the fact that it lies near so many more important anatomical features. For instance, it is at the approximate height of the beginning and end of the aortic arch. It is also approximately above the tracheal branch, where breath splits off into each lung. It is often used by doctors for such simple tasks as stethoscope placement.
The sternal angle's other name, the Angle of Louis, comes from its initial discoverer, Antoine Louis, a French physician who lived between 1723 and 1792. This feature, however, is not his only claim to fame. The original name for the guillotine was the louisette, named thus after its inventor, Antoine Louis. It was later renamed for its greatest proponent, Joseph Guillotin, a younger colleague of Antoine Louis.