Your spine consists of four primary curves
The cervical curve at the top of your spine consists of 7 vertebrae in a concave shape; the thoracic consists of 12 vertebrae in a convex shape; the lumbar 5 of the largest vertebrae in the spine forming a concave shape; and finally the sacral curve, consisting of 5 fused vertebrae and a convex shape. These curves link together to form the S-shape of your spine, enabling it to compress and expand in order to perform shock absorbing and movement duties. The essential shape of your spine is important for staying upright and preventing pain, but there are many forces working against you. Over time, the compressive nature of gravity, along with other factors, conspires to alter the curvature of your spine.