From Chapter 2, 'Spelling and Usage'

2.6   Plurals

Some nouns borrowed from foreign languages have only the regular English plural, e.g.:

(Greek) metropolis, metropolises

(Latin) campus, campuses; census, censuses; album, albums; forum, forums; museum, museums; premium, premiums

(Italian) canto, cantos; libretto, librettos; soprano, sopranos; sonata, sonatas

Some nouns, especially ones adopted from Greek and Latin, have only the foreign plural ending, e.g.:

(Greek) analysis, analyses; axis, axes; basis, bases; crisis, crises; diagnosis, diagnoses; oasis, oases; thesis, theses (and similarly with hypothesis, parenthesis, synthesis); criterion, criteria; phenomenon, phenomena

(Latin) alumnus, alumni; stimulus, stimuli; addendum, addenda; datum, data; desideratum, desiderata; erratum, errata; codex, codices

(German) lied, lieder

Other borrowed nouns may have either the English or the foreign plural. In general, the foreign plural is less common and more formal, or it may have a more specialized sense, as in these words of Greek or Latin origin:

formula (formulas in everyday usage, formulae in specialized contexts); thesaurus (thesauruses, thesauri); medium (mediums in spiritualism, media for (plural) means of communication); memorandum (memorandums, memoranda); referendum (referendums, referenda); ultimatum (ultimatums, ultimata); corpus (corpuses, corpora); appendix (appendixes for parts of the body, appendixes or appendices for additional parts of a publication); index (indexes for alphabetical lists of references, indices in mathematics)

Some adopted French words may retain the original plural -x, but -s is also found. The following forms are recommended:

adieu, adieus; milieu, milieux; tableau, tableaux

See also 8.4 on the plurals of foreign currencies.

No apostrophe should be used before the plural ending of abbreviations (where the -s follows any punctuation), names, numbers, letters, and words not normally used as nouns, e.g.:

MPs, POWs, Ph.D.s

the Henrys, the two Germanys

the 1960s, the twenties, ones and twos

as and es, the three Rs

haves and have nots

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