- Ultimate Guitar: This website provides guitar tabs and chords for songs, including Green Day tracks. The tabs often include notation for harmonies and backing vocals.
- Reddit: The subreddit r/greenday is a community of Green Day fans who often discuss the band's music. You may be able to find information about harmonies and backing vocals in Green Day songs by searching the subreddit or asking a question.
- Green Day official website: The official Green Day website sometimes includes liner notes or other information about their songs, which may include details about harmonies and backing vocals.