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 is a triangular shaped bone that lies between the lumbar vertebrae and the coccyx. Both sides of the sacrum attach the pelvis. The spine is made up of many parts. It is supported by an upright row of stacked spine bones, called vertebrae.