A single crystal undergoing plastic bending develops constraints due to the gradient in lateral contraction across the beam. These constraints result in lateral stresses which may be relieved by the process of anticlastic kinking. Anticlastic kink boundaries in rocksalt structure solids consist of arrays of {121}⟨110⟩ edge dislocations formed by the interaction of two systems of {110}⟨110⟩ glide dislocations, one system being responsible for slip in the main part of the crystal beam, the other confined to its corner.Temperature affects the structure of kink boundaries and their subsequent role in initiating fracture.I. At high temperatures (∼0.3Tm). The resultant edge dislocations in the boundary can move over their {121} slip planes and the kinks become sharp. The resultant dislocations are ineffective barriers to slip and the crystals are ductile.II. At low temperatures (∼0.1–0.2Tm). The resultant edge dislocations are immobile and the kinks consist of a diffuse array. The resultant dislocations provide strong barriers to slip and cracks nucleate at the kink boundary.III. At very low temperatures (∼0.1Tm). Fracture occurs before the second set of {110}⟨110⟩ glide dislocations have been activated to generate anticlastic kinks. Relaxation of the lateral stress results in a complex fracture.