The altas is the first cervical vertebrae, the first vertebrae period, of the spinal vertebrae. It sits next to the skull at the magnum foramen, forming the atlantooccipital joint (since the magnum foramen hole is part of the occipital bone). It is the joint which allows your head to shake "yes". It is labled as C1 in this picture and also has an arrow (as well as the atlantooccipital joint).