Relaxation is divided into normal and lateral, which may also be called parallel or tangential. In the case of normal relaxation the atomic structure of the upper layer is the same as in the bulk, but the distance between the top and second layers differs from the distance between the planes in the bulk (Fig. a). Pure normal relaxation is observed in metals. In most cases, it is the interlayer distance that is reduced first. For deeper layers, the deviation from the bulk interlayer distance decreases with depth, often in an oscillatory pattern. There may also be a uniform displacement of the upper layer parallel to the surface (Fig. b), which is called parallel or tangential relaxation. This occurs mostly on high index planes with low atom concentration.