The atlas, otherwise known as C1, is the first cervical vertebra, the uppermost bone in the spine.
It articulates with the skull superiorly, and with the Axis, C2 inferiorly. The joint between the atlas and the skull normally allows only nodding of the head. It does not under normal circumstances allow any rotatory movement, except at the extreme of neck rotation, where it will allow up to 3-4 degrees of rotation. It is not anatomically normal for this bone to remain rotated with respect to the skull when looking straight ahead.
The atlas articulates with the skull via the occipital condyles. The atlas bone’s articulations with the skull are considerably higher at the front than at the back.
Two images of the Atlas. Note that the superior facet sits higher at the front of the neck than at the back of the neck:
While it has been commonplace within the medical profession to claim that this joint between the skull and the atlas rarely gives rise to medical problems, it is in fact very straight forwards to perform a physical examination and confirm that in most adults it is prone to getting rotated out of place. The degree of rotation is variable, but can often be in the order of 20 degrees. It is more often rotated so that the left side of the atlas is rotated forwards. When the skull is rotated out of place on the atlas it skull sits crookedly on the neck.
Various names have been given to this phenomenon: upper cervical subluxation complex, atlas malalignment, atlas misalignment, atlanto-occipital subluxation, atlas subluxation, to name a few. For the purposes of simplicity I will refer to the problem as atlas subluxation, though what I am in fact referring to is the subluxation of the skull on the atlas. Probably the most anatomically correct name is atlanto-occipital subluxation.
A good image of the atlas and its function can be seen at this link: