1 Fish, 2 Fish (or is it 1 Fish, 2 Fishes?).

By Laura Schocker
Updated March 02, 2016
Silly geese
Credit: Melbye/Getty Images

At first it seems like a simple rule of English: When you have more than one of something, add an s. One apple. Two apples. In fact, language can be a whole lot more complicated. Two deer. Three teeth. Four mice.

“English is an organic language—it grows naturally by people using it,” says Kory Stamper, an associate editor for Merriam-Webster dictionaries. And as the language develops, irregularities are bound to crop up. “Sometimes there’s logic behind it and sometimes there’s not,” she says.

Some plural forms we just know. But others are trickier (Is it octopuses? Or octopi?). Here, a simplified guide to some of the most confusing plurals.

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Silly geese
Credit: Melbye/Getty Images

1 One Goose, Two Geese

Goose dates back about a thousand years ago to Old English, where the singular and plural words looked somewhat similar to “goose” and “geese,” which is how they’re treated in modern English. Efforts to regularize its plural form after the fact haven’t stuck. “Once you get something situated in its category, it’s very, very difficult to change it,” Stamper says.

Mongoose, on the other hand, is pluralized as mongooses.

2 One Moose, Two Moose

“Moose and goose might look alike in modern English but they come from two very different languages,” Stamper says—Moose is borrowed from Algonquian. “Because it comes from a different language, even though it looks like goose, we decided not to pluralize it as meese because there’s no connection there.”

Large game animals, like elk and deer, tend to get a “zero plural,” which means they’re the same in their singular and plural form. “Because moose were initially hunted, [moose] got grouped into that,” Stamper says.

3 One Octopus, Two Octopuses

If you say “octopuses,” someone in the room might correct you to octopi. Here’s why: “Octopus came into English in the 1600s. It was given a regular English plural,” Stamper says. So that’d be octopuses. But in the 17th and 18th centuries, grammarians started pushing to make English more like Latin—they looked for words borrowed from the language and started giving them their Latin plurals. Octopuses became octopi.

The problem? Octopus is actually borrowed from Greek. So then came a second round of scholars who corrected the plural back to its original octopuses. Today, either one is accepted, but octopuses is more common, Stamper says.

4 One Cul-de-Sac, Two Cul-de-Sacs or Culs-de-Sac

It made for one riveting Gilmore Girls discussion, but does anyone really know how to pluralize a cul-de-sac? Cul-de-sac is borrowed from French and is literally translated as bottom of the bag, according to Stamper. But unlike English, French tends to put the adjectives after the noun—so we say apple pie, and they say pie of apple.

The curious case here is that cul is the noun. Do you use the more French-ified plural, culs-de-sac, or the more English cul-de-sacs? Stamper says either one is correct. In technical, more formal writing, you tend to see culs-de-sac, while cul-de-sacs is more common in informal settings. The really good news? You rarely need to talk about more than one cul-de-sac. “I actually grew up on a cul-de-sac, and I never said cul-de-sacs,” Stamper says.

5 One Brother-in-Law, Two Brothers-in-Law

Interestingly, this word comes from a period of English history when primarily French kings ruled England, Stamper explains. A lot of emphasis was placed on government and law, and some new words were coined in English, but styled in French (with the adjective coming after the noun). The difference from cul-de-sacs, Stamper says, is that this time it’s much clearer that “brother” is the noun and “in-law” is an adjective. “Think about it this way: Since the men are primarily your brothers and the ‘-in-law’ part just further describes what kind of brothers they are, you make the important noun part plural,” says Mignon Fogarty, creator and host of the Grammar Girl podcast on the Quick and Dirty Tips network.

“The same holds true for other in-laws. They are your sisters-in-law, mothers-in-law, and so on.” Follow similar logic with attorney general, poet laureate, and the like—they pluralize to attorneys general and poets laureate, Stamper says.

6 One Passerby, Two Passersby

“You use the same kind of logic for ‘passersby’ as you do for ‘brothers-in-law,’ Fogarty says. “The people are passers, so you make that part plural. The ‘by’ part simply describes where they are passing.”

7 One Low Life, Two Low Lifes

Low life, in its singular version, arrived to the English language around the 1930s. “I can’t tell you why the plural is low lifes,” Stamper says. “Compound words get very weird in the way we pluralize them sometimes.” In the 1960s, some people tried to hyper-correct and change it to low lives, since the plural of life is lives—but that’s not right. So if someone calls you a “bunch of low lives,” you can at least feel smug that he or she is grammatically incorrect.

8 One Millennium, Two Millennia

When millennium first showed up in English, it was only used to describe a specific thousand-year period in which Christ was supposed to reign, Stamper explains. “There was only one of those—it didn’t have much plural use,” she says. Over time, people simply added an s: millenniums. But, like octopus, the classical grammarians came along and said that, because it’s a Latin word, the plural of millennium should be millennia. Both are “right,” but millennia is preferred, and more common. “People like plurals that make them sound smart,” she says.

9 One Scissors, Two Scissors

“Scissors is kind of like the word pants in that a pair of scissors is an object that we consider to be a composite plural,” Stamper says. Scissors came into English through French. And the French created the term through Latin, where the originating root word is actually singular and refers to a cutting implement, like a blade, she explains. “The French saw it as two cutting blades and pluralized it,” she says. “We talk about a pair of scissors, meaning one scissors.” So how to make it plural? It’s the same: scissors. “Scissors is this weird word that it looks plural and it can function as a singular word or a plural word,” Stamper says.

10 One Fish, Two Fish

“Fish is an old word,” Stamper says. Its original plural was fishes—and you still see that use, mostly in technical or scientific phrases. But in general use, fish is much more common as the plural of fish. The reason is probably the same as the game animal explanation for moose, deer, and elk. And it extends to specific types of fish, such as cod, tuna, and salmon. “The plural depends on your orientation to the object,” Stamper says. “If you are a fisher, the plural of fish is fish, the plural of cod is cod, and the plural of tuna is tuna. If you’re a scientist, the plural of fish can be fishes and cods and tunas. For most readers [it’s] the zero plural.”

11 One Do, Two Dos or Do’s

The dos and don’ts for “dos and don’ts” can be a little confusing. “Different style guides make different recommendations about how to make this phrase plural,” Fogarty says. The Chicago Manual of Style goes with dos and don’ts, while the Associated Press suggests do’s and don’ts. “If you're writing for a publication, make sure you know what style guide it follows, and if you're writing for yourself, pick the spelling that makes the most sense to you and use it consistently,” she says.