What do the ‘s’ in cats, the ‘en’ in oxen, and the vowel change in feet all have in common? On the surface, they seem completely unrelated. One is a consonant sound, one is a suffix, and one is a change in the middle of a word. Yet, in the world of linguistics, they share a deep connection: they are all doing the exact same job. They are all ways of making a noun plural.
This phenomenon, where a single unit of meaning takes on different forms, has a name: allomorphy. It’s one of the most fascinating and fundamental concepts in language, revealing the hidden logic behind so many of the “irregularities” we take for granted. Welcome to the world of the linguistic chameleon.
First, a Quick Refresher: What’s a Morpheme?
Before we can talk about allomorphs, we need to be clear on what a morpheme is. In simple terms, a morpheme is the smallest unit of meaning in a language. It can’t be broken down further without losing its meaning.
There are two main types:
- Free morphemes: These can stand alone as words. For example, cat, run, happy.
- Bound morphemes: These cannot stand alone and must be attached to another morpheme. Examples include prefixes like un- (as in unhappy) and suffixes like -ed (as in walked).
The plural marker in English is a bound morpheme. It carries the meaning of “more than one”, but it needs a word like ‘cat’ to attach to. Now, here’s where it gets interesting.
Meet the Allomorphs: The Different Faces of a Morpheme
An allomorph is a variant form of a morpheme. Think of the plural morpheme as a single “job” or “role” in the language: {PLURAL}. The different ways this job is performed—the ‘s’, the ‘en’, the vowel change—are its allomorphs. The term literally means “other form” (allo- “other” + -morph “form”).
These allomorphs are like chameleons. They change their appearance (their sound) to blend in with their environment (the sounds around them). This change isn’t random; it’s usually governed by predictable phonological rules.
The Classic Case: The English Plural ‘s’
The most common way to make a noun plural in English is to add an ‘s’. But have you ever paid close attention to how you pronounce that ‘s’? Say these words out loud:
- Cats
- Dogs
- Buses
If you listen carefully, you’ll notice you didn’t pronounce the ending the same way for all three. In fact, you naturally and unconsciously used three different allomorphs of the plural morpheme:
- The /s/ sound: In “cats”, the ‘s’ is pronounced as a voiceless /s/ sound (like the ‘s’ in “snake”). This happens when the noun ends in a voiceless consonant sound like /p/, /t/, /k/, or /f/.
Examples: pups, rocks, myths, laughs - The /z/ sound: In “dogs”, the ‘s’ is pronounced as a voiced /z/ sound (like the ‘z’ in “zoo”). This happens when the noun ends in a voiced consonant sound like /b/, /d/, /g/, /v/, or a vowel sound.
Examples: cubs, beds, bags, loves, boys, trees - The /ɪz/ or /əz/ sound: In “buses”, the ending is pronounced as a syllable containing a vowel and a /z/ sound. This happens when a noun ends in a “hissing” or “buzzing” sound (a sibilant) like /s/, /z/, /ʃ/ (sh), /ʒ/ (the ‘s’ in ‘measure’), /tʃ/ (ch), or /dʒ/ (j).
Examples: kisses, prizes, dishes, judges, churches
Why does this happen? It’s all about ease of pronunciation. Our vocal cords like to keep things simple. It’s much easier to produce a sequence of sounds that share the same voicing (both voiced or both voiceless). This is called assimilation. It’s also difficult to pronounce two very similar hissing sounds back-to-back, so our mouths cleverly insert a little vowel to break them up.
But What About the “Weird” Plurals?
So, the regular plural `-s` has three phonologically-conditioned allomorphs. What about words like oxen, feet, and sheep?
They are also allomorphs of the {PLURAL} morpheme! They just happen to be conditioned by the specific word (the lexicon) rather than the sound environment.
- The -en Suffix: This is a leftover from Old English. It used to be a much more common plural ending. Today, it survives in only a few words like oxen, children, and brethren.
- Vowel Mutation (Ablaut): The change from foot to feet or mouse to mice is another historical relic. This process, called ablaut or umlaut, was a regular feature of older Germanic languages.
- The Zero-Morph: What about one sheep, two sheep? Or one fish, two fish? Here, the plural is marked by… nothing! This is called a “zero-morph” or “zero allomorph.” The absence of a suffix is, itself, a way of signaling the plural for these specific words.
So, the full set of allomorphs for the English {PLURAL} morpheme includes /s/, /z/, /ɪz/, /-en/, vowel mutation, and the zero-morph. They all perform the same function: meaning “more than one.”
Allomorphy is Everywhere!
Once you know what to look for, you’ll see allomorphy all over English.
The Past Tense -ed
The regular past tense morpheme, written as -ed, works exactly like the plural -s. Say these words aloud:
- Walked: Pronounced with a /t/ sound because /k/ is voiceless.
- Jogged: Pronounced with a /d/ sound because /g/ is voiced.
- Wanted: Pronounced with an /ɪd/ or /əd/ sound because the verb stem ends in /t/. (Try saying “wantd” without the extra vowel!)
The morpheme is {PAST TENSE}, and its main allomorphs are /t/, /d/, and /ɪd/.
The Negative Prefix *in-*
The prefix that means “not” or “the opposite of” also changes its shape to match the sound that follows it:
- in-direct, in-tolerant (The basic form)
- im-possible, im-balance (Before /p/ or /b/, it changes to /m/ to match the place of articulation—both are made with the lips)
- il-legal, il-logical (It assimilates to the following /l/)
- ir-regular, ir-responsible (It assimilates to the following /r/)
Why Does This Matter?
Understanding allomorphy isn’t just a party trick for linguists. It has real-world implications.
For the linguist, it reveals the beautifully systematic and rule-governed nature of language that hides just beneath the surface. It shows how phonology (the sound system) and morphology (the word-formation system) are deeply intertwined.
For the language learner, it can be a huge lightbulb moment. Instead of memorizing hundreds of seemingly random pronunciation quirks, you can learn the underlying rule once. Knowing why “cats” ends with an /s/ sound while “dogs” ends with a /z/ sound helps you pronounce any new plural noun correctly, even one you’ve never seen before. It transforms exceptions into examples of a different kind of rule, making the language feel less chaotic and more logical.
So next time you hear someone say “fishes” or “judges” or “walked”, take a moment to appreciate the linguistic chameleon at work. You’re not just hearing a word; you’re hearing a tiny, elegant, and automatic adjustment—an allomorph perfectly suited to its environment.