One of the fundamental tenets of chemistry is the law of definite proportions. It states that the ratio of the elements which make up a compound is an invariable simple fraction. This proportion was called stoichiometry. The law of definite proportions was universally accepted after the famous debate between Proust and Berthollet, which was unequivocally won (as it seemed at that time) by Proust. This law became a theoretical basis of Dalton’s atomistic hypothesis and was a stimulus for rapid progress in chemistry, especially organic. More than 100 years passed, and at the beginning of the twentieth century Kurnakov showed  that intermetallic compounds do not necessarily have definite stoichiometric compositions. After that, deviation from stoichiometry was experimentally observed in various classes of crystalline solids (oxides, sulfides, etc.).