"The atlas is the spinal vertebra (C1) closest to the skull. It sits next to the foramen magnum, the hole in the occipital bone. The ""atlanto-occipital joint"" is the connection between the atlas and the occipital bone, and is stabilized by ligaments. ""Atlanto-occipital overlapping"" (AOO) is characterized by a decreased distance between the atlas and the occipital bone. In some cases, the dorsal arch of the atlas may actually protrude into the foramen magnum. See the image (courtesy of: www.wikispaces.com)."
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