Consider how the curved GB in Figure 24.11, moves; the classical picture for the change in energy as an atom changes its position is shown in Figure 24.12, but reality is clearly much more complex. This diagram is drawn to show the (100) surface of MgO. IF the curved interface is actually normal to the plane of the figure, the GB is pure tilt in character. It is clear from Figure 24.12 that not all the ions move in the same direction. If you select different ions in Figure 24.11 you can appreciate that some move.