<prompt> Extentsion of neck occurs atAtlanto-occipital jointC4-C5 veebraAtlantoaxial jointC6-C7  <response>: 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 