The ribs are curved, flat bones forming most of the thoracic cage. Typically, each person has 12 pairs of ribs. However, the number of ribs is variable, and there can be only eleven or even thirteen ribs on one side. If more ribs are present than usual, they are usually formed in either the cervical region or lumbar region. These ribs are known as supernumerary ribs.

The ribs usually increase in size and length from rib 1 to rib 7, and they decrease from rib 7 to rib 12. All ribs form several articulations. Their proximal ends directly articulate with the vertebrae of the spine, forming the costovertebral joints and costotransverse joints. In contrast, the distal ends of only the first seven ribs directly articulate with the sternum.

According to their articulation with the sternum, all ribs can be grouped into three types:

True (vertebrosternal) ribs - ribs 1 to 7; they attach directly to the sternum via their own costal cartilages;
False (vertebrochondral) ribs - ribs 8 to 10; they are connected to the sternum indirectly via the cartilage of the rib above them; all costal cartilages of the false ribs join with the costal cartilage of the seventh rib;
Floating (free) ribs - ribs 11 and 12; these ribs do not articulate with other ribs, adjacent costal cartilages or sternum; their costal cartilages tend to end within the abdominal musculature.

Ribs can be classified in one more way. Each rib can either be typical rib or atypical rib based on its anatomical features.

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Ribs