The line where two surfaces meet Example: This tetrahedron has 6 edges. It can also be the boundary of a plane shape, such as the circumference of a circle.
1 12 thoracic vertebrae in the upper back corresponding to each pair of ribs. 2  5 lumbar vertebrae in the lower back.  5 sacral vertebrae which are fused together to form 1 bone called the sacrum and 4 coccygeal vertebrae that are also fused together to form the coccyx or tailbone.
Back pain diagnosis often includes the name of the part of the spine that is involved in causing the pain. Learn about the parts of the spine. The spine is made of 24 individual vertebrae, that go from the skull to the sacrum. The sacrum bone - which is really 5 bones that begin to fuse at about age one, with the fusion complete approximately by age 30 - plus the coccyx complete the spine at the bottom. (The coccyx is your tailbone, and is the very last bone of the spine.) The spine connects to the ribs and pelvis to make the trunk or core.