Because English is a language with very heterogeneous origins, many English words derived from French, Greek or Latin have irregular plurals or even two different plurals. Some words which may appear plural are in fact singular, as in the various names of areas of knowledge. Furthermore, the use of words denoting groups and partitive expressions often affects whether the related verb is singular or plural. Finally, some nouns, called attributive nouns, are used in singular or plural form in front of other nouns to modify these.