<unk> s are four atypical ribs (two lowermost pairs, XI-XII) in the human ribcage. They are called so because they are attached to the vertebrae only, and not to the sternum or cartilage coming off of the sternum. Some people are missing one of the two pairs.  Others have a third pair.  Most, however, possess only two pairs.
Their position can be permanently altered by a form of body modification called tightlacing, which uses a corset to compress and move the ribs.
Floating rib
Cervical rib
Sternocostal surface
Subcostal plane
Diaphragmatic surface of heart
Mediastinal surface of lung
Sternal plane
Mediastinal pleura