During the past few months of this year, as in those same months of any year, we’ve been hearing a great deal of Christmas music. Some of the songs in the mix — “Rudolph the Red-Nosed Reindeer,” “Have Yourself a Merry Little Christmas,” “The Christmas Song” — few of us have ever known a time without, and others make it in because of their seasonally themed lyrical content. But certain songs just sound like Christmas songs, somehow, and to understand what, in musical terms, fills those compositions with the spirit of the holiday season, watch the five-minute Vox explainer above that reveals “the secret chord that makes Christmas music sound so Christmassy.”

First we should distinguish popular Christmas songs from popular non-Christmas songs, especially ones recorded in the past half-century. “Rock ’n’ roll songs (and the subsequent pop songs influenced by the genre) may only contain three or four chords, each chord usually being just a major or a minor — the two chord ‘flavors’ analogous to chocolate and vanilla,” writes Slate’s Adam Ragusea. In contrast, a selection from “the Great American Songbook” might “use a Baskin-Robbins shop full of chords and chord flavors — 7^ths and 9^ths, half and fully diminished, various inversions, and more” under melodies that “tend to include a lot of chromatic notes (the black notes on the piano when playing in the key of C major).”

In the era when most beloved Christmas standards were conceived, songwriters still made much use of that wide musical palette, the sonic colors of which had as much to do with jazz as with pop. But since the 1960s, writers of pop songs  have used these now-exotic harmonies “to get a ‘classic’ sound. For instance, John Lennon and Yoko Ono’s ‘Happy Xmas (War Is Over)’ includes some notes in its choral parts that I think are intended to recall the harmonic vocabulary of those 1940s Christmas standards.” No coincidence, surely, that Mariah Carey’s “All I Want for Christmas Is You,” perhaps the only Christmas song written in recent decades to attain the same popularity as the old standards, uses the same compositional techniques.

“I count at least 13 distinct chords at work in ‘All I Want for Christmas Is You,’ resulting in a sumptuously chromatic melody,” writes Ragusea. “The song also includes what I consider the most Christmassy chord of all — a minor subdominant, or “iv,” chord with an added 6, under the words ‘underneath the Christmas tree,’ among other places.” As in Irving Berlin’s “White Christmas,” he notes, “the chord comes immediately after a major subdominant chord, giving the effect of a ‘bright’ major subdominant that you might say ‘sighs’ or ‘melts’ into a ‘dark’ minor subdominant spiked with a ‘spicy’ extra tone (the added 6), before the songs settle back into their tonic, or ‘home,’ chords.” And so we come to the unexpected finding — though hardly a displeasing one — that a properly made Christmas song has more than a little in common with a properly made Christmas cocktail.