Cephalometric landmarks and variables used for analysis of the head and neck posture.Ba, basion; N. nasion; S, sella; C0. Base of the occiput; C1, the posterior arch of the atlas; C2, the spinous process of the second vertebra; C2sp, the most superior-posterior point on the body of the second vertebra; C2ip, the most inferior-posterior point on the body of the second vertebra; C3ia, the most inferior-anterior point on the body of the third vertebra; C4ip, the most inferior-posterior point on the body of the fourth vertebra; OPT, posterior tangent to the odontoid process through C2ip; CVT, posterior tangent to the odontoid process through inferior C4ip.