Rib
Ribs are classed as flat bones which usually have a protective role in the body. Humans have 24 ribs, in 12 pairs. All are attached at the back to the thoracic vertebrae, and are numbered from 1–12 according to the vertebrae they attach to. The first rib is attached to thoracic vertebra 1 (T1). At the front of the body most of the ribs are joined by costal cartilages to the sternum. The ribs connect to the vertebrae with two joints, the costovertebral joints.
Costotransverse joint
This costotransverse joint is present in all but the eleventh and twelfth ribs. The first ten ribs have two joints in close proximity posteriorly; the costovertebral joints and the costotranseverse joints. This arrangement restrains the motion of the ribs allowing them to work in a parallel fashion during breathing. If a typical rib had only one joint posteriorly the resultant swivel action would allow a rib to be non-parallel with respect to the neighboring ribs making for a very inefficient breathing.