The work of a homeopathic student while studying and applying materia medica is one to constantly compare and differentiate. He must compare the pathogenesis of a remedy with the recorded anamnesis of the patient; he must differentiate the apparently similar symptoms of two or more medicinal agents in order to select the simillimum. To enable the student or practitioner to do this correctly the student must have proper knowledge of the individuality of the remedy; something that is peculiar, uncommon, or sufficiently characteristic in the polychrest remedy that may be used as a pivotal point of comparison. This pivotal point may be a ‘keynote,’ a ‘characteristic,’ or the ‘red strand of the rope.’ This excellent work by Dr H.C. Allen helps the reader to grasp the PQRS symptoms in no time.