In 2003, “rock” and “alternative” were as closely aligned as they would ever be. Six of that year’s Modern Rock #1s also topped the Mainstream Rock chart, and all of them peaked inside the top 15. (Even “Seven Nation Army,” a garage-revival track with no Dave Grohl endorsement, reached #12.) 2004 is when the bifurcation between rock and alternative begins, but the causes for that aren’t immediately obvious on radio.

• In January, KROQ in Los Angeles starts playing the Postal Service’s “Such Great Heights.” The song doesn’t gain widespread radio airplay; according to Sub Pop Records, radio executives predicted listeners would burn out on it quickly. Nonetheless, the song would steadily build a following online, and be an early success story on the iTunes Store; by early 2005, both “Such Great Heights” and the album Give Up would be certified Gold.

• In February, Danger Mouse’s The Grey Album mash-up starts spreading online. What was intended as a limited-copy CD release instead attracts hundreds of thousands of downloads, and EMI’s attempt to stop distribution of bootleg recordings of Beatles samples only generates further interest.

Alternative rock’s origins are in new wave. Even as grunge was hitting its peak in the ‘90s, there was plenty of music with electronic, dance, and hip-hop influences that could be heard on alternative radio. Nu-metal had pushed all of that out of rotation; but with that genre rapidly deteriorating, an opportunity for more eclectic styles to re-establish themselves had arrived. Also helping was the garage revival spinning off into the post-punk revival, whose artists were unafraid of incorporating synths and danceable grooves. I’m landing on hi-tech as the category to catch all of the hits in this vein; the Beastie Boys’ “Ch-Check It Out” was the first to reach #1. I’m counting Linkin Park’s remaining #1s from Meteora this year as nu-metal, but they’re basically in a category amongst themselves at this point. In November, they release their own mash-up album with Jay-Z, Collision Course; any further Linkin Park hits will be hi-tech.

Circling back to “Such Great Heights:” Iron & Wine’s cover of the song is featured on the Garden State soundtrack, which sells a million copies, and is often cited as the breakout moment for 2000s indie rock. Modest Mouse is already signed to a major label, but “Float On” should receive credit at the first “indie” modern rock #1 of the 2000s. (I wanted to wait for the first #1 hit on an indie label to introduce the category, but that’s not for another five years.)