What is a 864 chord in music theory?

In music theory, an "864 chord" does not exist. However, there are several music theory concepts and chords names that might sound similar. It's possible that you may be referring to one of these:

1. "Augmented Sixth Chord" (Aug6 Chord):

- An augmented sixth chord, abbreviated as "Aug6," involves a "chordal structure" built with augmented sixth intervals. While it can also be represented as "86," it is more commonly referred to as an augmented sixth chord or abbreviated as Aug6.

2. "Power Chord - Fifth Power and Octave":

- On some diagrams for power chords, there's notation like "5 6 4" to indicate playing only the root (fifth fret for power chords), power fifth (sixth fret), and octave or root an octave up (fourth fret). So sometimes it is described as an "864" power chord due to the fret positions, but this is quite informal terminology.