Technically, extended chords include 9th, 11th and 13th intervals, but it’s easier to think of 6ths and 7ths in the same way too. All these chords add notes to basic major or minor chords (C, Dm and so on), ‘extending’ them with more colourful sounds (such as Cmaj7, Dm9). Jeff Buckley’s 1994 album Grace featured dozens of extended chords. Also listen to Amy Winehouse’s Octobersong. The maj7#11 chord is loved by shred gurus Joe Satriani and Steve Vai. G13 sounds great in soul, jazz and blues.