The most beloved Christmas songs are often the jazziest. Holiday mainstays like “White Christmas,” “The Christmas Song,” and even Mariah Carey’s “All I Want for Christmas Is You” all borrow from early 20th-century jazz, and as such, tend to feature similar chords. One of those chords, Slate points out, is particularly Christmassy: the strange-sounding “minor subdominant” chord with an added sixth. Even stranger is the fact that our brains are wired not to like it.

As Adam Ragusea, a journalism professor at Mercer University and former music scholar writes in Slate, the minor subdominant chord with an added sixth is used in many classic Christmas songs, including Carey’s inescapable hit. He describes the sound as “melting,” like snow before a fireplace. It’s a lovely sound in context, but its weird melange of notes suggests that we should interpret it as dissonant, or bad-sounding (consonant chords, in contrast, are obviously pleasant to the ear). Scientists have discovered that our brains don’t particularly like those chords, so it is especially odd that they have made their way into one of the most beloved genres of music.

First, a quick music theory refresher: Chords are sets of three or more notes played together, and often those notes are all within the same scale. The first note of a “subdominant” chord is the subdominant, or fourth note, of a particular scale. In music theory-speak, the “dominant” note is the fifth note of a scale, so the subdominant one is the fourth, or one below it. The “minor” element means dropping the third note down a semitone, giving a chord a sad sound. Play that chord together with the sixth note of the scale, and you’ve got yourself a subdominant minor with an added sixth.

This is not, by any estimation, a chord you encounter very often. These notes are a strange pairing, and together they sound unusual — so odd, in fact, that it wouldn’t be a stretch to call them dissonant. Why these kinds of chords are “bad”-sounding has vexed musicians and scientists alike for ages: Are they somehow objectively bad-sounding, suggesting that there is some hardwired standard for a “good”-sounding chord? Or is that standard culturally defined?

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A more thorough explanation will probably combine the two ideas, but musicologists at least know that certain pairs of notes produce some off-putting physiological effects. When two notes that are close together (that is, close in frequency) on a scale are played simultaneously — say, the first and second notes of a scale — they produce a feeling referred to as “beating.” We end up hearing a single tone rising and falling in loudness, which in our ears sounds like a wah-wah-wah sound. Some musical genres like this — take the atonal gamelan music of Indonesia — while others, like most European classical music, generally try to avoid it. In the same way humans learn to like the caustic burn of alcohol, some also learn to appreciate the discord that dissonance causes in our brains.

Dissonant-sounding jazz chords were not embraced by early audiences, just as the weird-sounding note combinations of Bach, Beethoven, and Mozart were initially met with some hesitation. These composers, like Irving Berlin and Mariah Carey, didn’t give a hoot. They knew that a chord, however dissonant sounding, may sound different in context anyway: In “White Christmas,” the subdominant minor with a sixth caps off the phrase ending with “and children listen,” and in “All I Want for Christmas Is You,” it underscores the line “I don’t care about the presents / Underneath the Christmas tree.” In neither case does the chord sound jarring — instead, it just sounds like Christmas.