Taylor Swift and Jack Antonoff are listed as songwriters on Olivia Rodrigo's new album 'Sour' - here's why

• Olivia Rodrigo released her highly anticipated debut album, "Sour," on Friday.
• The fourth track, "1 Step Forward, 3 Steps Back," interpolates Taylor Swift's "New Year's Day."
• Rodrigo's song repurposes the memorable piano riff from Swift's "Reputation" closer.

Taylor Swift has a songwriting credit on Olivia Rodrigo's new album, but not for the reason you might think.

Rodrigo's highly anticipated debut, "Sour," was released to rave reviews on Friday.

Fans quickly noticed that Swift and her frequent collaborator, Jack Antonoff, are both listed as cowriters on the album's fourth track, "1 Steps Forward, 3 Steps Back."

This is because Rodrigo, who is a noted Swiftie, repurposed the memorable piano riff from Swift's "New Year's Day," the final track on 2017's "Reputation."

By all accounts, Rodrigo's song interpolates "New Year's Day" rather than samples it directly, which means she re-recorded the piano line - either by herself or with her own studio musicians - instead of copy-and-pasting the original recording.

Typically, interpolating a song only requires permission from whoever wrote the composition, not the legal owner of the song's master. This is important, since a private-equity company owns the original recording of "New Year's Day," not Swift.

Rodrigo managed to retain control of her own master recordings when she signed with Geffen/Interscope in 2020. She said she was inspired by Swift's fight to own her music.

A post shared by Olivia Rodrigo (@oliviarodrigo)

Fittingly, "1 Steps Forward, 3 Steps Back" contains a sneaky Easter egg for fellow Swifties. The common idiom is actually "one step forward, two steps back," but Rodrigo wrote "three" instead, likely because 13 is Swift's lucky number.

Swift has cosigned Rodrigo's music and even sent her a handwritten letter earlier this year, so perhaps a true songwriting collaboration is on the table for the future.