First, the typology of the various sounds employed in a language: in the case of vowels, for example, the opposition between simple and double vowels (lengthened as in ā, ō; or diphthongized as in ai); among simple vowels, the opposition between those that are pure (a, u) and those that are modified (e, ); among those that are pure, there are those that are susceptible of various pronunciations (such as o), and those that have only one (a, u); finally, among this last group, some are subject to change and can receive an umlaut (a and u); the, on the other hand, always remains the same.