What are irregular plural nouns?

What is an irregular plural noun?

Most nouns that you’re familiar with end in the letter s, such as lion and lions, hat and hats, and plum and plums.

But some words simply don’t adhere to this basic rule. These words can include person vs. people, man vs. men, or knife vs. knives. These are known as irregular plural nouns, which are nouns that do not become plural by adding -s or -es.

Write with Confidence using Editor

Elevate your writing with real-time, intelligent assistance

Learn more

Common examples of irregular plural nouns

Like most grammatical rules, you’ve likely used the following examples of these singular and plural nouns in sentences and writing before. As you can see, these words don’t just tack on an “s” at the end. They undergo a transformation that can seem quirky and puzzling at first glance:

• child and children
• foot and feet
• goose and geese
• calf and calves
• leaf and leaves
• cactus and cacti
• focus and foci
• thesis and theses
• mouse and mice

Why are there irregular plural nouns in the English language?

Irregular plurals exist because the English language has a rich history of borrowing words and linguistic evolution. Many of these irregular plurals can be traced back to Old English or even earlier sources such as Germanic and Romance languages. Over time, while most words adopted the straightforward -s plural, these irregular nouns clung to their ancient forms.

Simple rules for irregular plural nouns

Fortunately for writers, there are certain irregular plural nouns that adhere to a rule. This makes it easier to recognize when a singular noun needs more attention to be turned into a plural noun.

Nouns that end in -f

Words that end in -f or -fe are irregular when made plural. To change a singular noun into a plural noun, replace the -f (or -fe) ending with a -ves. The following examples fit this mold:

• knife and knives
• life and lives
• wife and wives
• calf and calves
• leaf and leaves

Of course, there are always exceptions to any rule. For example, roof and roofs and proof and proofs end regularly. Just more quirks of the English language!

Nouns that end in -o

Nouns that end in -o require the letters -es to become plural, instead of just the letter -s. Replace the -o ending with -es, such as in these examples:

• potato and potatoes
• tomato and tomatoes
• hero and heroes
• torpedo and torpedoes
• veto and vetoes

Again, there are exceptions. Some words ending in -o are borrowed from other languages (such as Italian), and they take only an -s to make a plural like piano and pianos, canto and cantos, photo and photos. The word zero can be pluralized as zeros and zeroes, depending on the style guide that you use.

Nouns that change vowels

Certain nouns with vowels in the middle of the word simply receive a replacement for another vowel to become plural, instead of tacking on letters to the end. The most common usage of this rule includes nouns that have -oo in the middle, which get changed to -ee. Examples include:

• foot and feet
• tooth and teeth
• goose and geese
• man and men
• woman and women

Irregular nouns that are the same both singular and plural

Sometimes you’ll come across a noun for which there is no singular form because just one word is versatile enough to refer to one or many of something. The following irregular nouns, usually pertaining to animals, illustrate this point:

• sheep
• fish
• moose
• swine
• buffalo
• shrimp
• deer
• trout

Very different irregular plurals

Here is another exception to the rules: because English has evolved over the centuries from a mishmash of languages, these irregular plural nouns play by their own rules.

• mouse and mice
• louse and lice
• die and dice
• ox and oxen
• child and children
• person and people

Irregular plural nouns might seem confusing at first, but with practice and familiarity, you can grasp this and other grammatical rules and writing tips in the English language—such as how to use quotation marks correctly, create coordinating conjunctions, and understand what assonance is.