You're probably going to hear the phrase "merry Christmas" a lot over the next few days. But it might not always sound the same.

Americans vary in how they pronounce the words "merry," "marry," and "Mary."

A majority, 57%, of Americans pronounce all three words the same.

The divide highlights something fascinating about American English.

You're probably going to hear the phrase "merry Christmas" a lot over the next few days.

But depending on who you talk to, it might not sound the same every time.

That's because how people pronounce the word "merry" is at the heart of one of the most interesting divides in American English. And diving into the data reveals a fascinating quirk about our language.

How do you pronounce 'merry,' 'marry,' and 'Mary'?

Think of how you pronounce "merry," "marry," and "Mary."

Do you pronounce all those words the same? Do you pronounce two of them the same, and one different? Or do they all sound different when you say them?

If you chose option 1, you're in the majority — 57% of the country pronounces all three words the same. That's according to the Harvard Dialect Study, a legendary 2003 survey of more than 30,000 Americans by linguist Bert Vaux and data scientist Scott Golder.

Related: 'Soda,' 'pop,' or 'coke': More than 400,000 Americans weighed in, and a map of their answers is exactly what you'd expect

If you're one of that 57%, odds are that all three words rhyme with "hairy" when you say them. It's a phenomenon known as "vowel merging," in which distinct vowel sounds in a dialect slowly converge into the same sound over time.

Meanwhile, 17% of the country pronounces all three words differently. These are the people whose dialects have resisted vowel merging. If you're in that group, you most likely say "merry" with a "meh" sound at the beginning, rhyme Mary with "hairy," and pronounce "marry" with the same vowel sound in "trap."

The final 26% of respondents pronounce two of the three words the same, in most cases "marry" and "Mary," with "merry" being the odd one out. You can see the whole results here.

Interestingly, pronouncing all three words differently can reveal where you come from: It's mostly associated with the Northeast, especially Philadelphia, the New York City area, and Boston.

The explanation hinges on the letter R

For New Yorkers and Bostonians, the reason for the distinction likely has to do with their famous tendency of dropping the R sound at the ends of syllables — "pahk the cah in Hahvahd Yahd" is an example.

The presence of an R shapes the way we articulate the vowel before it, San Diego State University linguistics professor Aaron Dinkin told Business Insider. So speakers who treat their R's differently are going to wind up with two different pronunciations of the same word.

Depending on where you're from, you might consider the R sound in "merry" to be at the end of the first syllable rather than the start of the second. But if your dialect shies away from placing R's at the end of syllables, you'll place the sound at the start of the second syllable, allowing for less influence on the vowel before it. The rules don't just apply to merry and marry, but any similarly-structured words too, like "very," "fairy," or "Harry."

Over in Philadelphia, speakers have something entirely different going on: they tend to pronounce "merry" the same as "Murray," another example of how sounds in English are constantly shifting.

"People learn the dialect features of the community they grow up in, for the most part, and every dialect is suitable for the communicative needs of the people that speak it," Dinkin told Business Insider.

"But changes are always taking place as one generation learns the dialect ever-so-slightly differently than the previous generation did."