Milewski's typology is a language classification system proposed in the 1960s by the Polish linguist Tadeusz Milewski. In this classification active and tripartite languages were omitted because they were little known in the study of linguistics at that time. Milewski proposed a division of languages into 6 groups, based upon consideration of 4 main syntactic relationships; these were: These criteria are interesting from a typological point of view because in many languages there is no difference between the sentence and the nominal phrase. Milewski's typology can be employed to analyze languages with case marking but can also be used with those that use a fixed word order or a specific form of incorporation. For simplicity, the table below classifies casual languages in which the nominal attribute is marked with the genitive case. The letters a, b, and c represent formal inflective markers specific to each language. For instance, "a" always represents the formal marker that signifies the experiencer, called either the "nominative" or the "absolutive" depending upon whether this morpheme marks the agent of the action (as in nominative–accusative languages) or the patient (as in ergative–absolutive languages). As the table shows: