“Thinking Out Loud” is Sheeran’s smash hit about lifelong love. Though he was just 23 when he wrote it, he imagines the love lasting well into his later years.

The message resonated with fans, and it shattered several records—it was the first song to spend a full year in the UK Top 40, and the first song to reach 500 million streams on Spotify (the service also deemed it the top “first dance” song in the world).

Sheeran wrote the song about his then-girlfriend Athina Andrelos. He calls it the only happy song on the album x, on which he strived for a more serious vibe, working extensively with Rick Rubin.

Sheeran co-wrote “Thinking Out Loud” with Welsh folk singer Amy Wadge, and his friend and long-time collaborator Jake Gosling produced the track. Chris Leonard engineered it, and played guitar.

The bass and drums are very close to Marvin Gaye’s “Let’s Get It On"—Sheeran has even mixed a little of the "Let’s Get It On” lyrics into his life performances: