Examples of such spontaneous energy minimizing changes to GB character include the rotation of the GB plane to achieve equilibrium at triple junctions -both those formed by the intersection of three GBs within a polycrystal and those formed between a GB and two free surfaces at the exterior of a microstructure. It is this phenomenon that is exploited to produce the quarter-and half-loop bicrystal geometries used in classic constant-driving-force GB migration experiments . The inclination of the GB plane can also change during GB migration , or when faceting into different combinations of GB planes to reduce energy , or upon the disappearance of a grain during grain growth (which requires establishment of new equilibrium configurations at the newly formed triple junctions). This class of energy minimizing changes to GB character constitute paths through the GB energy landscape that are restricted to two-dimensional submanifolds (the GB plane fundamental zone (FZ) for a fixed misorientation).