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, depending on usage. Some words which may appear plural are in fact singular, as in the various names of areas of knowledge. Furthermore, the use of collective nouns and partitive expressions often affects whether the related verb is singular or plural. Finally, despite some notable exceptions, nouns used as adjectives normally revert to their singular form.

7.1 Words with unusual plural forms

Some frequent examples in our institutional context are campus, curriculum, practicum and thesis, which take the plural forms campuses, curricula, practicums and theses, respectively. Because the standard English plural -s is becoming increasingly widespread, in those cases where you can choose between two alternative plural forms this guide recommends the -s form (for example, prefer syllabuses to syllabi). For a longer list of unusual plural forms, see Unusual plural forms in Appendix I: Lists of additional information.