The Hall-Petch equation was originally based on yield-point measurements in low-carbon. It has been found to express the grain-size dependence of the flow stress at any plastic out to ductile fracture and also to express the varistion of brittle fracture stress with size and the dependence of fatigue strength on grain size. The Hall-Petch equation also has been found to apply not only to grain boundaries but to other kinds of boundaries such as ferite-cementite in pearlite, mechanical twins,and martensite plates.