Irregular Plurals From Mice and Geese to Cacti and Criteria

Ask a child for the plural of mouse and you might hear mouses. It is a sensible guess: English adds -s or -es to almost every noun. But a few hundred nouns break that rule, and many of them are common words you need every day. Luckily, most irregular plurals follow a handful of patterns. Learn those and you only have to memorise the true oddballs.

The regular rules first

To understand what is irregular, start with what is regular:

• Most nouns add -s: word → words, tile → tiles.
• Nouns ending in s, x, z, ch, sh add -es: bus → buses, box → boxes, quiz → quizzes, match → matches, wish → wishes.
• Nouns ending in a consonant plus y change Y to -ies: city → cities, puppy → puppies. After a vowel, just add -s: day → days, key → keys.

Everything else in this lesson is an exception to these three rules.

Pattern 1: vowel changes

A small group of very old English words form their plurals by changing the vowel inside the word:

Do not extend the pattern to similar-looking words. The plural of moose is moose, not "meese", and a mongoose becomes mongooses.

Pattern 2: -en plurals

Three common words keep an old -en ending:

• child → children
• ox → oxen
• brother → brethren (only for members of a religious group or society; family brothers are brothers)

Pattern 3: -f and -fe become -ves

Many nouns ending in F or FE change to -ves:

leaf → leaves, loaf → loaves, half → halves, knife → knives, wife → wives, life → lives, wolf → wolves, thief → thieves, shelf → shelves, calf → calves

But not all of them. These keep the F and just add -s:

roof → roofs, chief → chiefs, belief → beliefs, cliff → cliffs, chef → chefs, proof → proofs

A few accept both: scarfs or scarves, hoofs or hooves, dwarfs or dwarves. All of these are valid in standard dictionaries, so choose one and stay consistent.

Pattern 4: -o words

Nouns ending in O are unpredictable:

• Add -es: potato → potatoes, tomato → tomatoes, hero → heroes, echo → echoes, veto → vetoes
• Add -s: piano → pianos, photo → photos, radio → radios, zoo → zoos, video → videos
• Either: zero → zeros/zeroes, volcano → volcanoes/volcanos, mosquito → mosquitoes/mosquitos

A useful hint: shortened words (photo from photograph, piano from pianoforte) and words with a vowel before the O (radio, zoo) usually take plain -s.

Pattern 5: same singular and plural

Some nouns do not change at all:

sheep, deer, moose, fish, series, species, aircraft, offspring

Fish has a twist: fishes is used when you mean several different species (the fishes of the Amazon).

Pattern 6: Latin and Greek plurals

English has borrowed many words from Latin and Greek, and some kept their original plurals.

Many of these also accept a regular English plural: cactuses, funguses, formulas, antennas, indexes, appendixes. In everyday writing the English form is often more natural; in science and academic writing the classical form is still common. Note that antennae is normal for insects, while radios have antennas.

Compound nouns

For compounds written as separate or hyphenated words, pluralise the main noun: mothers-in-law, passers-by, attorneys general. For compounds written as one word, pluralise the end: spoonfuls, toothbrushes.

Why plurals matter in word games

Plurals are the easiest way to hook a word in Scrabble: adding an S to the end of a word on the board lets you play a second word at right angles. Irregular plurals matter because they cannot be hooked that way. You cannot add an S to MICE or TEETH, and SHEEP takes no S at all. On the other hand, the Latin plurals give you useful extra words: CACTI, FUNGI and RADII are all valid plays. Check any plural in the word checker before you commit.

Study the plurals

The flashcards below cover fifteen irregular plurals across all six patterns. Say the plural before you flip, and read the note on the back for related words.

Practice quiz