Irregular Plurals: the Rules Behind Them
Most English nouns pluralize with a simple added s, but a stubborn set of irregular plurals resists that pattern entirely, preserving older, historically layered forms — some inherited from Old English sound changes, some borrowed wholesale from Latin and Greek, and some that have simply never regularized despite centuries of pressure from the dominant -s pattern — and understanding the different historical sources behind these irregularities explains why the exceptions cluster the way they do.
Old English vowel-mutation plurals
Foot/feet, tooth/teeth, goose/geese, man/men, woman/women, and mouse/mice all descend from a single historical process called i-mutation, in which an old plural ending (since lost) shifted the root vowel forward in the mouth before the ending itself disappeared, leaving the shifted vowel as the only surviving signal of plurality. These are among the oldest and most deeply entrenched irregular plurals in English, surviving essentially unchanged for over a thousand years despite constant pressure from the regular -s pattern that has since swallowed up most other originally irregular nouns.
Old English -en plurals
Child/children and ox/oxen preserve a different, separate historical plural pattern using an -en ending, once far more common in Old English than it is today; most nouns that once used this pattern have since regularized to the standard -s ending, leaving only a small handful of survivors. Children is itself a doubly marked plural, historically combining an older -er plural (childer, still found in some regional English dialects) with the additional -en ending, making it an unusually layered irregular form even among English's already irregular plurals.
Unchanged plurals (zero plurals)
Sheep, deer, fish, moose, and several other nouns, largely referring to animals, use an identical form for both singular and plural: one sheep, ten sheep. This pattern is thought to relate partly to these nouns' traditional use in hunting and herding contexts, where a group was conceptually treated more like an undifferentiated mass than a count of individuals, though the exact historical explanation for why this specific pattern survives mainly among animal names remains debated among linguists.
Borrowed Latin and Greek plurals
Words borrowed from Latin and Greek often retain their original language's plural pattern rather than adopting English's regular -s: datum/data, criterion/criteria, phenomenon/phenomena, cactus/cacti (though 'cactuses' is now widely accepted too), analysis/analyses. Many of these borrowed plurals are gradually regularizing in everyday usage even while their original forms remain standard or preferred in academic and formal writing, creating a genuine, ongoing tension between historical correctness and everyday usage for several of these words.
Compound nouns and where the plural marker goes
Irregular pluralization interacts with compound nouns in its own specific way: when a compound noun's core meaning sits in its first element, the plural marker typically attaches there rather than at the very end: mothers-in-law, passersby, attorneys general. This differs from the simpler pattern of ordinary compound nouns, where the plural marker attaches to the final element as usual (bookshelves, not bookshelfs), and getting this backward is a fairly common error even among careful writers unfamiliar with the specific compound in question.
Words that look plural but function as singular, or vice versa
A separate category of irregularity involves nouns whose surface form doesn't match their actual grammatical number: news, which looks plural but takes a singular verb (The news is good), and scissors, which looks and functions as plural despite referring to one physical object (These scissors are sharp). These mismatches between surface form and grammatical behavior are a distinct kind of irregularity from the vowel-mutation and borrowed-plural patterns above, rooted in different historical quirks specific to each individual word.
Why English hasn't regularized these forms over time
Linguists studying irregular forms generally find that a word's frequency in everyday use strongly predicts whether an irregular pattern survives or eventually regularizes — highly frequent words like foot, tooth, man, woman, and child are used often enough that their irregular plurals stay deeply entrenched in speakers' memory, resisting the pull toward the regular -s pattern that has swallowed up most less frequently used irregular nouns over the centuries.
How irregular plurals are acquired by children learning English
Language-acquisition research consistently shows that children learning English initially overregularize irregular plurals, producing forms like 'foots' or 'mans' before eventually learning the correct irregular forms through repeated exposure and correction — this overregularization pattern is considered strong evidence that children are actively extracting and applying the general -s pluralization rule productively, rather than simply imitating adult speech word for word, and the eventual correction reflects memorized exceptions overriding the more general, productive rule.
Irregular plurals in compound and figurative usage
Irregular plurals typically stay irregular even in figurative or compound uses far removed from their literal meaning: sawteeth, gear teeth, a law with real teeth all use 'teeth,' never a regularized 'tooths,' even though the objects being described (saws, gears, laws) have nothing literally in common with mouths. This consistency across figurative extensions is a useful diagnostic showing how deeply entrenched these particular irregular forms are in the mental lexicon, essentially immune to regularization pressure even in contexts far removed from the word's original literal meaning.
Regularization in progress: borrowed plurals in flux
Several borrowed Latin and Greek plurals are currently in active transition toward regularized English forms, with both versions considered acceptable depending on register: cactus/cacti versus cactuses, index/indices versus indexes, syllabus/syllabi versus syllabuses. Formal academic and scientific writing tends to prefer the historically original plural, while everyday and informal writing increasingly favors the regularized English form, illustrating a live, ongoing linguistic change rather than a settled rule.
A comparative look at how other languages handle irregular plurals
English's irregular plural system is comparatively modest next to languages with much more elaborate irregular declension systems, such as German or Arabic, where plural formation for many nouns follows patterns that resist easy generalization far more thoroughly than English's relatively small set of exceptions. This comparative context is useful for understanding that English's irregular plurals, while genuinely irregular relative to its own dominant -s pattern, represent a fairly light irregular burden by the standards of many other well-studied world languages.
A practical takeaway for writers and learners
Because irregular plurals resist derivation from any single rule, the most reliable approach remains direct memorization, ideally organized by the historical categories covered above (vowel-mutation plurals, -en plurals, zero plurals, borrowed plurals) rather than as one undifferentiated list, since grouping by shared origin tends to make the patterns easier to retain and recognize in new, unfamiliar words that might belong to the same historical class.
A final organizing summary of the main categories
Vowel-mutation plurals (foot/feet, tooth/teeth, goose/geese, man/men, mouse/mice) descend from ancient sound changes. The -en plurals (child/children, ox/oxen) preserve an older, now largely abandoned pluralization pattern. Zero plurals (sheep, deer, fish) use identical singular and plural forms. Borrowed plurals (data, criteria, phenomena) retain Latin or Greek endings, sometimes alongside a regularizing English alternative. Keeping these four categories distinct makes the whole irregular-plural system considerably easier to reason about than treating it as one undifferentiated list of exceptions. Keeping a short personal reference list of the trickiest borrowed and compound plurals is a practical habit for any writer who edits formal documents regularly.
Frequently asked questions
Why do 'foot' and 'tooth' have irregular plurals but 'boot' and 'root' don't?
'Foot' and 'tooth' descend from a specific class of Old English nouns affected by i-mutation, an ancient vowel-shifting sound change, while 'boot' and 'root' either entered English later or belonged to a different noun class not affected by that same historical process, so they simply took the regular -s ending instead.
Is 'octopi' the correct plural of octopus?
Not historically — 'octopus' actually comes from Greek, not Latin, so the Latin-style '-i' plural (octopi) is a mistaken analogy rather than the historically accurate form; 'octopuses' is the standard English plural, and 'octopodes' is the rarely used, more historically faithful Greek-derived plural.
Are irregular plurals disappearing from English over time?
The oldest and most frequently used irregular plurals (feet, teeth, mice, men, women, children) show no real sign of regularizing, since their high frequency keeps them firmly entrenched in speakers' memory, while some less common borrowed plurals (like cactus/cacti versus cactuses) are gradually shifting toward regular forms in everyday usage.
Why does 'sheep' stay the same in both singular and plural?
This 'zero plural' pattern is thought to relate to how these animals were traditionally referred to in herding and hunting contexts, where the group was conceptually treated as an undifferentiated mass rather than a count of separate individuals, though linguists don't have one single fully settled explanation for why this pattern survives specifically among certain animal names.
Is it 'attorneys general' or 'attorney generals'?
'Attorneys general' is the traditionally correct form, pluralizing the core noun 'attorney' rather than the modifying adjective 'general,' following the same principle as other compound nouns where the plural marker attaches to the element carrying the core meaning rather than automatically going at the very end. The more casual 'attorney generals' does appear in everyday usage, but careful formal and legal writing favors 'attorneys general.'