Classification systems, help the logical and systematic separation and organization of knowledge about the disease so that one may reason from the signs and symptoms seen in the patient to a presumed etiologic history. From the presumed etiologic history to identification of the condition and from there to the prescribed course of treatment.The names of a disease are a convenient way of stating briefly the endpoint of a diagnostic process that progresses from the assessment of symptoms and signs towards knowledge of causation. In order to obtain more knowledge about the causation of periodontitis and to be able to discuss cases between colleagues, the various forms of the disease have to be classified. Since periodontitis must be regarded as a syndrome with a complex etiology, classifications of periodontitis should be based on the nominalistic concept.