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.
The sacrum and coccyx form the inferior end of the vertebral column where it meets the hip bones to form the pelvis. The triangular sacrum forms joints between the lumbar vertebrae and the hip bones. It also contains many passages for the spinal nerves that exit the spinal cord and spread through the pelvis and legs.