Extentsion of neck occurs at

C1 and C2 form a unique set of aiculations that provide a great degree of mobility for the skull. C1 serves as a ring or washer that the skull rests upon and aiculates in a pivot joint with the dens or odontoid process of C2. Approximately 50% of flexion extension of the neck happens between the occiput and C1; 50% of the rotation of the neck happens between C1 and C2. Ref - medscape.com