Ever wonder why a lot of the big hit songs on the radio sound the same? That's because there are only so many combinations of chords you can string together to make music. As a songwriter, I'm always analyzing chord progressions, and lately I've been hearing a lot of one in particular: minor 6-4-1-5. Forget your daily lotto. These four numbers can make a musician a lot of money. I've found that anytime this sequence of chords forms a chorus, the song is a smash. To prove it, I ensconced myself in my upstate New York toolshed, where I stayed for 12 days, subsisting on only Toblerone bars and pouches of Capri Sun.

With Project 6-4-1-5 under way, I began with my theory's bell cow: Sarah McLachlan's gigantic 1997 hit, "Building a Mystery." Using the chord progression minor 6-4-1-5 for its chorus, it is one of the most harmonically influential songs written in the last decade. I say harmonically because the sound of McLachlan's refrain is unforgettable, yet I can't really follow the lyrics. (I think the story takes place in the Bat Cave, but I'm not certain.)

Though McLachlan didn't invent this chord progression -- it appears two years earlier in Joan Osborne's "One of Us" -- it was only after she used minor 6-4-1-5 in her megahit that it began to show up elsewhere. Lee Ann Womack borrowed it to great effect three years later for the chorus of her hit country ballad, "I Hope You Dance." These two songs serve as great replacements for one another in a pinch at the karaoke bar.

The most recent 6-4-1-5 subscriber is Ryan Cabrera, with "On the Way Down," which I instantly pegged as a "Mystery" devotee. Ditto for 2001's Michelle Branch ditty "All You Wanted." Avril Lavigne's biggest song to date, 2002's "Complicated," also has the same chord progression. She liked it so much, she used it again on "Don't Tell Me" two years later. All these tunes burned up top-40 radio, and though you may not remember a single lyric, the bet here is that upon second listen you could easily hum their refrains.

So what is it about the chord progression minor 6-4-1-5 that makes it so infectious? Music theorists will tell you that the wavelengths of each chord fit together mathematically. But I just think people know a hit tune when they hear it.

What Do You Think?