The sternum is not attached to the last two pairs of ribs at the bottom of the rib cage.
These ribs are known as "floating ribs" because their only attachment is at the rear of the rib cage, where they are connected to the spine's vertebrae.
Floating ribs are the 11th and 12th pairs of ribs.
The floating ribs are smaller than the rest of the ribs and feature as cartilaginous tips.