Final answer:
The atlanto-occipital joint is formed between the occipital condyles of the skull and the superior articular processes of the atlas (C1 vertebra). It allows for flexion and extension of the head and has limited rotational movement. The joint plays an important role in moving the head up and down.
Step-by-step explanation:
The atlanto-occipital joint is formed by the articulations between the occipital condyles of the skull and the superior articular processes of the atlas (C1 vertebra). This joint allows the skull to rock forward and backward, producing flexion and extension of the head, which moves the head up and down as when shaking your head "yes." The orientation of the joint largely prohibits rotation but allows for considerable extension, flexion, and lateral flexion.