39/Smooth is the very first album of Green Day. Which most, but not all, of their fans now don't even know about.
It was combined with EPs 1,000 Hours and Slappy to make 1,039/Smoothed Out Slappy Hours, which is also not that known but is getting more known because they play a lot of their songs from that album again.
Person 1: Hey, let's listen to At the Library by Green Day.
Person 2: I didn't know they had a song like that. From what album?
Person 1: It's from 39/Smooth- their first album.
Person 2: Huh? I thought American Idiot was their first album.
Person 3: I thought 21st Century Breakdown was.
Person 1: Well, now you know it isn't.