It has been increasingly realized in recent years that many problems relating to the fundamental properties of the metallic state can best be approached through experimental investigations on specimens consisting of single crystals of metals of known purity. It has been demonstrated, for example, that a metal crystal shows no deviation from Hooke’s Law until a definite stress is reached, when permanent deformation or fracture takes place by glide on definite crystallographic planes. The yield point, from this point of view, is the least stress at which permanent deformation can be detected; the value of the yield point obtained in any determination depends on the sensitiveness of the method used for detecting permanent deformation.