One sheep, two sheep. One fish, two fish. One deer, two deer. From childhood rhymes to everyday speech, we navigate these curious plurals without a second thought. But have you ever stopped to wonder why? Why do we say “two cats” and “two dogs,” but not “two sheeps”? Why does adding that simple ‘-s’ feel so wrong for certain words?

This isn’t a grammatical error or a random exception. It’s a linguistic phenomenon known as zero-marking or the zero plural. It’s a ghost in our grammatical machine, an invisible mark of plurality that tells a fascinating story about the history of the English language, how we perceive the world, and how languages evolve.

What Exactly is a Zero Plural?

In linguistics, a morpheme is the smallest unit of meaning in a language. The word “cats” has two morphemes: “cat” (the furry animal) and “-s” (a plural marker meaning “more than one”). The “-s” is by far the most common way to form plurals in English. It’s a productive, powerful rule that we apply to new words automatically. If we invent a new gadget called a “wug,” we instinctively know that two of them are “wugs.”

A zero-marked plural, however, uses a zero morpheme. This means that the plural is signaled by the absence of any change. The plural form is identical to the singular form.

• One sheep / A flock of sheep
• One aircraft / Three aircraft
• One series / Several series
• One moose / A herd of moose

The plurality isn’t lost; it’s simply understood from the context—the numbers, articles, or verbs surrounding the noun. “The sheep is in the field” (singular) versus “The sheep are in the field” (plural). So, why do these particular words get to play by different rules?

Echoes of Old English

The primary reason for many of our most common zero plurals is history. Modern English is a descendant of Old English, a Germanic language with a much more complex grammatical system. While we mainly rely on word order today, Old English used a case system (like modern German or Latin) where nouns changed their endings based on their function in a sentence (subject, object, etc.) and their grammatical gender (masculine, feminine, neuter).

Nouns were grouped into different classes, or declensions, each with its own set of endings. The “-s” plural we use today comes from the dominant declension: the strong masculine nouns. Over time, as the case system fell away, this “-s” ending was generalized and applied to most other nouns.

However, some nouns from other declensions held on to their old forms. A key group was the neuter a-stem nouns. In Old English, words like scēap (sheep), dēor (animal, which later narrowed to mean deer), and swīn (swine/pig) had the same form in the nominative singular and plural. They never took an ‘-s’ to begin with. When the language simplified, they just… stayed that way. They are linguistic fossils, preserving a grammatical pattern that has otherwise been dead for nearly a thousand years.

Nouns of the Hunt and Herd

While the Old English explanation is the grammatical root, there’s a compelling semantic pattern to these words. Notice how many of them are animals, particularly animals that are hunted, fished, or herded:

• deer
• moose
• elk
• grouse
• quail
• fish
• trout
• salmon

Linguists suggest this isn’t a coincidence. These words often refer to animals viewed as a collective or a mass, rather than as individuals. A hunter goes out to hunt deer (the species, the concept) not necessarily to count “one deer, two deers”. A farmer raises sheep as a flock. This conceptualization of the animals as a group may have helped preserve the historical zero plural. It just “felt” right to treat them as an undifferentiated whole.

This contrasts with domesticated animals we interact with more individually, like dogs, cats, and horses, which all take a standard “-s” plural.

Beyond the Barnyard: Loanwords and Measurement

Zero-marking isn’t confined to Old English survivors. The phenomenon also appears in other contexts, often involving words borrowed from other languages.

Many words from Japanese, for instance, retain their original lack of a plural marker when adopted into English. We talk about two samurai, a team of ninja, or several plates of sushi. While “ninjas” and “samurais” are gaining traction (that powerful “-s” rule is always at work!), the zero plural is still very common.

The same applies to some words from French that already end in an ‘s’ sound, like chassis or corps (as in the Marine Corps), where adding another ‘s’ would be phonetically awkward.

We also see a kind of temporary zero-marking with units of measurement, especially when used as a compound adjective. We say someone is “six foot tall” (using foot as part of the adjective “six-foot”) but that they have “two feet” (using feet as a simple noun). The same goes for a “ten-ton truck” but “twenty tons of gravel”.

The Gray Areas: When Rules Get Fuzzy

Language is rarely neat, and zero plurals have their own fascinating ambiguities. Take our old friend, fish.

You can say, “I caught three fish.” Here, you’re using the zero plural, likely referring to multiple fish of the same species. But you can also say, “The fishes of the Caribbean are beautiful.” In biology and ichthyology, “fishes” is used to refer to multiple different species of fish. It’s a subtle but useful distinction.

A similar logic applies to people. “People” is the irregular plural of “person.” But “peoples” is also a word, used to refer to distinct ethnic, cultural, or national groups. “The indigenous peoples of North America” refers to the many different nations, not just a crowd of individuals.

These examples show how language can repurpose an old pattern to create new layers of meaning.

The Invisible Plural’s Lasting Lesson

The next time you hear “two sheep” or “a school of fish”, you’re hearing more than just a quirky exception. You’re hearing an echo from a bygone era of English, a time of different grammatical rules and structures. Zero-marking is a testament to the fact that language is a living archeological site, with ancient foundations still visible beneath a modern surface.

These invisible plurals show us that language isn’t just a set of arbitrary rules but a system shaped by history, perception, and practicality. They are not mistakes; they are monuments to the rich, messy, and fascinating journey of our language.