One of the two lowest ribs on each side, which are not attached to the breastbone.
The 11th and 12th ribs, which do not attach to the sternum.
The two lower pairs of ribs are unique as they are connected to the spine at the back but do not attach to the sternum (breastbone) through cartilage, unlike the rest of the ribs.
The final two ribs on each side that have their anterior ends unattached to the sternum.