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 triangle-shaped bone located between the fifth lumbar vertebra and the coccyx that consists of five vertebrae fused together. The sacrum joins on each side with the bones of the pelvis.