The question of flys vs flies comes up whenever someone writes about insects, aviation, or the simple act of moving through the air. Most of the time, flies is correct. Flys exists in a narrow technical sense, but if you're reaching for the plural of "fly" or the third-person present tense of the verb, flys will mark your writing as an error.
The rule: when a word ends in a consonant plus "y," change "y" to "ies"
English has a consistent rule for words ending in a consonant followed by "y": the "y" changes to "ies" in the plural and in the third-person singular present tense. The word "fly" ends in "l" plus "y," so both the noun plural and the verb conjugation follow this pattern:
- Noun plural: one fly, two flies
- Verb (third person): she flies, he flies, it flies
- Simple past: she flew (irregular, not "flied" in most senses)
The same rule applies to "cry" (cries), "try" (tries), "carry" (carries), and dozens of other verbs and nouns. It's not an exception for "fly"; it's a pattern the word shares with much of the language.
When "flied" appears, and what about "flys"?
Baseball introduces a wrinkle. In baseball, a "fly ball" is a ball hit high into the air, and the accepted past tense in that context is flied: "He flied out to center field." Using "flew" there would confuse a baseball audience, so the sport has regularised the verb. Outside baseball, flew is the standard past tense of "fly."
As for flys, it does appear in one narrow technical area: theatre. Stage rigging systems use the term "fly" for the overhead space above the stage where scenery is raised, and the plural "flys" can refer to those rigging systems collectively. It's a specialist term. In everyday writing, flys is not recognised as a correct form by any major dictionary.
So the summary is this: write flies unless you're writing for a technical stagecraft context. Flys is not the plural of the insect, and it's not how the verb conjugates.
Common sentences where the confusion surfaces
The mix-up tends to show up in three types of sentences. The first is the noun plural: "There were flies everywhere" gets misspelled as "There were flys everywhere." The second is the verb: "She flies to New York every Monday" becomes "She flys to New York every Monday." The third involves compound nouns like "horseflies" or "mayflies," where the same spelling rule applies. None of these forms take a "y" ending.
A quick check: if you can swap the word for "tries" or "cries" and the sentence still sounds grammatically normal, the correct spelling ends in "ies," not "ys."
Why the mistake happens
Writers who misspell this word are usually applying the wrong analogy. English nouns that end in a vowel plus "y" simply add "s" to form the plural: "day" becomes "days," "key" becomes "keys," "boy" becomes "boys." If you're used to that pattern, adding just an "s" to "fly" feels natural. It isn't. The consonant-plus-"y" rule breaks the pattern, and that's where the confusion starts.
It's a similar mechanism to the mix-up between tore vs torn, where writers apply the wrong template because a more familiar one feels closer. The fix is the same: recognise which rule actually governs the word, not just which rule feels familiar.
Quick reference: flys vs flies
If you need a fast answer before hitting send:
- Flies is the correct plural of the insect "fly."
- Flies is the correct third-person singular present tense of the verb "fly."
- Flys belongs only in technical theatre writing about stage rigging.
- Flied is acceptable past tense only in baseball contexts.
- Flew is the standard past tense everywhere else.
Spelling accuracy in small words like this matters more than it might seem. Loser vs looser is another example where a single-letter difference changes how a reader perceives the writer's credibility. Getting "flies" right costs nothing and signals precision.
The Merriam-Webster entry for "fly" confirms all principal parts: fly, flew, flown, flying, flies. Not a "flys" in sight.