Plastic deformation of crystalline materials usually occurs by slip, which is the sliding of planes of atoms slide over one another (Figure 8.1). The planes on which slip occurs are called slip planes and the directions of the shear are the slip directions. These are crystallographic planes and directions, and are characteristic of the crystal structure. The magnitude of the shear displacement by slip is an integral number of inter-atomic distances, so that the lattice is left unaltered by slip. If slip occurs on only a part of a plane, there remains a boundary between the slipped and unslipped portions of the plane, which is called a dislocation. It is the motion of these dislocations that cause slip.