Why It’s ‘Feet’ Not ‘Foots’: The Logic of I-Umlaut

If you have ever tried to teach English to a child or a non-native speaker, you have inevitably stumbled into the trap of the “irregular” plural. It usually goes something like this:

“One cat, two cats. One dog, two dogs. One foot, two… foots?”

“No”, you correct them gently. “Two feet.”

“Okay”, the learner nods. “So, one boot, two beet?”

“No, two boots.”

At this point, the learner usually throws their hands up in despair, convinced that English is a language built on chaos and anarchy. And on the surface, they are right. Why do we change the vowel in “foot” to get “feet”, but simply add an ‘s’ to “boot”? Is it just a random quirk we have to memorize?

The answer is no. It isn’t random. In fact, it is the result of a strict, logical, and biological rule of pronunciation that occurred over 1,500 years ago. To understand it, we have to dig into the history of Old English and a phenomenon linguists call i-umlaut (or i-mutation).

The Ancestor of “Foot”: Proto-Germanic

To solve the mystery of the shifting vowels, we have to travel back in time—before Modern English, before Shakespeare, and even before Beowulf. We need to go back to Proto-Germanic, the reconstruction of the language spoken by the tribes in Northern Europe around 500 BCE, which eventually split into English, German, Dutch, and the Scandinavian languages.

In those days, the word for “foot” was likely fōt (pronounced with a long ‘o’, similar to ‘vote’).

However, Proto-Germanic was a language that relied heavily on suffixes (endings attached to words) to indicate grammar. The plural ending wasn’t just ‘s’. To make fōt plural, speakers added the suffix -iz. So, the plural of “foot” was:

*fōtiz

This is where the magic begins. Say that word out loud: fōtiz. Notice what your mouth does. The first vowel (ō) is a “back vowel”, meaning your tongue is pulled back in your mouth. The second vowel (i) in the suffix is a “front vowel”, meaning your tongue pushes forward and high against the roof of your mouth.

The Mechanics of Laziness: Anticipatory Assimilation

Human beings are efficient creatures, especially when it comes to speaking. We don’t like moving our tongues back and forth drastically within a single word if we can avoid it. In linguistics, this leads to a process called assimilation.

As early Germanic speakers said fōtiz, their brains started looking ahead. While they were pronouncing the ‘o’ in the root, their tongues were already inching forward to get ready for the ‘i’ in the suffix. They were anticipating the next sound.

Eventually, this anticipation changed the sound of the root vowel. The back vowel ‘o’ was pulled forward to become a front vowel, which sounded more like ‘e’ (specifically, a long ‘e’ sound, like in the French fête).

• Original: fōtiz
• Middle Stage (Umlaut): fētiz

This specific type of change—where a back vowel fronts because of an ‘i’ or ‘j’ sound in the following syllable—is what linguists call i-umlaut (pronounced oom-lout). The word comes from German, meaning “sound alteration.”

The Case of the Disappearing Suffix

If the story stopped there, the plural of foot would be “feeties.” But language never stops changing. As Old English evolved, it developed a habit of eroding the ends of words. Speakers stopped stressing the final syllables, and over centuries, those helpful grammatical suffixes withered away.

First, the final ‘z’ dropped off. Then, the ‘i’ weakened to a generic “uh” sound (a schwa), and finally, it disappeared entirely. But here is the catch: the damage to the root vowel was already done.

Even though the -iz suffix (the “criminal” that caused the change) fled the scene, the evidence remained. The root vowel had permanently shifted from ‘o’ to ‘e’.

• Proto-Germanic: fōt (singular) / fōtiz (plural)
• Umlaut Era: fōt / fētiz
• Old English: fōt / fēt

And thus, we were left with an “irregular” plural. The trigger for the change was lost to history, leaving behind a word that changes its vowel for seemingly no reason.

The Great Vowel Shift

You might be wondering, “Wait, if it shifted to an ‘e’ sound, why do we pronounce it ‘feet’ (with an ‘ee’ sound) today?”

That is due to a much later event called the Great Vowel Shift, which happened between 1400 and 1700. During this time, almost all long vowels in English moved higher in the mouth.

• The Old English fēt (pronounced like ‘fate’) shifted upward to become modern feet.
• The Old English singular fōt (pronounced like ‘vote’ or ‘boat’) shifted upward to become foot.

The “Mutant” Family: Other Victims of I-Umlaut

“Foot” and “feet” aren’t the only words that underwent this process. Once you understand i-umlaut, you start seeing it everywhere in English grammar. It explains almost all of our bizarre vowel-shifting plurals.

1. Mouse and Mice

The Proto-Germanic word for mouse was mūs. The plural was mūsiz. The ‘u’ (a back vowel) was pulled forward by the ‘i’ to become a ‘y’ sound (like the German ‘ü’). Eventually, the suffix dropped, leaving mūs vs. mȳs. During the Great Vowel Shift, that long ‘y’ sound evolved into the diphthong /ai/ we use today in “mice.”

2. Tooth and Teeth

Just like foot, this started as tanth vs. tanthiz. Through a series of complex shifts (loss of the ‘n’, lengthening of the vowel), it followed the same ō to ē path as foot/feet.

3. Goose and Geese

Proto-Germanic gans vs. gansiz followed a similar trajectory. The ‘a’ sound was fronted and raised, eventually leading to the modern ‘ee’ sound.

4. Man and Men

This is perhaps the most common one. The plural of mann was manniz. If you try to say “manniz” quickly, you will naturally find yourself saying “menniz.” The ‘a’ raises to ‘e’ very easily in anticipation of the high ‘i’. When the suffix dropped, we were left with man/men.

So, Why “Boots”?

We return to the original question. If foot becomes feet, why doesn’t boot become beet?

The answer is chronology. I-umlaut was a distinct historical event. It happened during a specific window of time in the Germanic languages (roughly 450–600 CE) and then it stopped being a “productive” rule.

The word “boot” didn’t exist in Old English. It entered the language much later, likely around the 14th century from Old French (bote) or Old Norse (bóti). By the time “boot” arrived in English, the machinery of i-umlaut had been turned off for centuries. The automatic process of changing vowels to match suffixes no longer happened.

Therefore, “boot” simply adopted the standard pluralization rule of the time: adding an -s.

The Hidden Logic of Language

What makes linguistics so fascinating is that what appears to be a mistake or an irregularity is usually a fossil of a perfectly logical system that no longer exists.

The plural of “feet” is not an exception to the rule; it is a survivor. It is a linguistic echo of an ancient speaker, thousands of years ago, whose tongue got a little bit lazy while trying to pronounce a plural ending that we haven’t written in a millennium.

So, the next time you correct a learner for saying “foots”, you can tell them that they aren’t necessarily being illogical—they’re just a few thousand years too late!