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.
Your spine is made up of 33 irregularly shaped bones called vertebrae. Each vertebra has a hole in the middle through which the spinal cord runs. The spinal cord can be divided into five different regions, from top to bottom: Your ribs attach to your 12 thoracic vertebrae. 2  Your five sturdy lumbar vertebrae carry most of the weight of your upper body and provide a stable centre of gravity when you move. 3  Your sacrum is made up of five fused vertebrae.