Every year, the songs that become staples on the American Top 40 charts have a tone. In 2008, that tone was pure optimism, with songs like Katy Perry’s “I Kissed a Girl” and Colbie Caillat’s “Bubbly.” In 2015, the songs were minor-key and kind of frantic: songs like Taylor Swift’s “Blank Space,” and Hozier’s “Take Me to Church.” This year, the songs are even more similar: They all sound like songs for dancing and crying, songs whose uncertainty could go on for a minute or for 40.

What’s so interesting about 2016’s pop hits, though, is that so many songs with this tone (and there are a lot of them) get there through the same exact music theory principle.

I first noticed the 2016 tone in the Chainsmokers’ “Closer,” featuring Halsey, when it played immediately after Justin Bieber’s “Sorry” on a playlist I was creating. (Like other songs mentioned in this piece, “Sorry” was technically released in 2015, but it remained at or near the top of the charts for a good chunk of this year, which makes it a 2016 hit for the purposes of this exercise.) Despite being performed, written, and produced by entirely different people, there was something in the chord structure that made the two songs sound the same.

I couldn’t figure it out, so I called Owen Pallett and asked him to listen to both. Pallett is a brilliant pop music theorist who is a frequent Arcade Fire collaborator and an art-pop auteur. “The one thing I noticed,” he told me, “is that they have ambiguous key centers.”

What’s an ambiguous key center?

Usually in popular music, it’s incredibly clear which chord is the “home base” in any song. Normally a song’s key center is the last note or first note in every four-bar loop. Even the most basic music theory training would make it very easy to recognize the key of a song on Top 40 radio on first listen.

Take the Beatles’ “She Loves You” as an example: In the chorus, when they sing “She loves you / yeah, yeah, yeah / With a love like that / you know you should be gla-ad” — the key of the song is the note that “glad” ends on, G major.

Without getting too technical: A key is defined by the note that the scale you’re using starts on, and consists of all seven notes in that scale. Of course, most songs use more than one chord, but they always return to the principal chord of the song’s key. In the case of the Beatles’ “She Loves You,” that key is G. Typically, a major key is more upbeat and positive. It sounds comfortable and expected. A minor key sounds more dissonant and potentially sad.

Normally it’s obvious which key a pop song belongs in. But in a song with an ambiguous key center, the chords are arranged in such a way that it’s difficult to tell which chord is the home base. Sometimes this is achieved by never actually playing the note the song is based on, as in Charlie Puth’s “We Don’t Talk Anymore,” featuring Selena Gomez. In this case, the song uses the chords A, B, C-sharp minor, and G-sharp minor, chords that could belong in three different keys (E, C-sharp minor, or G-sharp minor). Sonically, though, the song sounds most like it’s in E major.

Or sometimes it happens because there are only a few chords in the song but two of them sound like they could be the key center, like in DNCE’s “Cake by the Ocean,” which uses E minor and A minor back and forth over and over again.

“It sounds more intellectual than it is when you hear it,” said Ethan Hein, an adjunct professor of music and doctoral student at Montclair State and NYU, when I asked him about all this. “All it means when you hear it is that you’re just kind of floating and drifting. The songs are a little more open-ended.”

Ambiguous key centers aren’t new, they’ve just become more popular

The chords and notes used in 2016’s biggest hits aren’t unique; they’re the same exact ones we’ve been hearing since the invention of modern music around 1750. But by shifting a few notes here and there, current pop artists can keep their songs from sounding predictable and give them a more open-ended tone. Instead of landing back on the chord we expect it to, these songs lift back up into another note, making the song more buoyant, surprising, and dynamic.

Even ambiguous key centers aren’t a new thing, per se. Ambiguous key centers were very common chord structures for classic rock songs. That’s because the biggest influence on classic rock was the blues, which still had key centers, but the rules were totally different. And that, both Hine and Pallett agree, plays a big role in why we are seeing the emergence of so many ambiguous pop songs right now.

“This music, it tends to start in black culture, and then young white people and young hipsters kind of make it its own. That’s going back to jazz,” Hine said. “That’s kind of been the history of American music. And it’s definitely what’s happening here.”

Inspired by hip-hop and the rise of less formulaic popular music by R&B singers and producers, white artists have begun (intentionally or unintentionally) creating songs that mimic that music within their own genre. A great song by Solange Knowles or even a song like Mark Ronson’s “Uptown Funk” has less predictability than a standard pop song because it draws on jazz music, which, while predictable in its own way, has an entirely different setup.

This has been happening for a little while in modern popular music. Katy Perry’s “Teenage Dream” (2008) denied listeners the root key to create a sense of weightlessness. And Daft Punk’s “Get Lucky” (2014) was completely unclear on what its key even was.

“It’s interesting because, at least in the history of Western art song, there’s never been nearly as much of a preference toward ambiguous key centers as there is in current pop music,” Pallett told me.

Ambiguous key centers make songs emotionally dynamic

What all of the ambiguous pop songs of 2016 are doing is using the major key and its relative minor to keep the listener guessing about whether the song is happy (major) or sad (minor).

A perfect example of this is Major Lazer’s “Cold Water” featuring Justin Bieber, which could either be in F-sharp minor or A major. Though there’s a backing EDM-inspired beat to this song, Justin Bieber’s voice sounds more somber and serious than uplifting. Take, for example, a couplet in the first verse: “So take a deep breath and let it go / You shouldn’t be drowning on your own.” It’s about the sweetness of reckless love, right? Or is it about a destructive relationship?

“This is one of those things where I firmly believe that this kind of analysis is like a biopsy,” Pallett noted. “Whoever wrote the chords and the melody, I don’t think that they were consciously, like, sitting down to create an ambiguous key center.”

He’s right. It’s totally possible all of these artists ended up at these song feelings individually and unintentionally.

“I do feel like it’s really hard to communicate with millions of different people, and if you’re doing it successfully, it does mean that you’ve really tapped into something,” Hein told me. “And it does feel like everyone tapped into this one thing. There’s something bigger than any artist or listener here.”

In 2016, the big unifying factor of white pop was the ambiguous key structure: a sonic space that sounds buoyant and light, and isn’t quite sure if it’s happy or sad. That waffling — between happiness and sadness, between fear and joy, between major and minor — does seem incredibly apt in this year. If 2016 taught us anything, it’s that our country is deeply divided on almost every issue. For every political or emotional event that happened this year, it seems like half of America felt happy and the other felt sad. It’s pop music’s job to appeal to both of them, and by using ambiguous key centers, these artists managed to do just that.

Here’s a playlist of songs with ambiguous key centers that landed in the Top 40 this year: