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 lowest part of the spine is called the lumbar spine. This area usually has 5 vertebrae. However, sometimes people are born with 6 vertebrae in the lumbar region. The base of your spine (called the sacrum) is a group of specialized vertebrae that connects the spine to the pelvis. When one of the bones forms as a vertebra rather than part of the sacrum, it is called a transitional (or sixth) vertebra.