When the head is moved from side to side, the first vertebra (the atlas) pivots around the dens or odontoid process of the second vertebra (the axis).
The atlas (C1) is the first cervical vertebra and sits directly under the skull. The axis (C2) is the second cervical vertebra and has a bony protuberance called the dens or odontoid process that protrudes upward from its body. The atlas pivots around the dens of the axis to allow for rotational movement of the head, such as when shaking the head "no." This joint is known as the atlantoaxial joint.