The Petrushka chord is a frequently used polychord used in Igor Stravinsky's ballet Petrushka. These two major triads, C major and F# major - a tritone apart, these two chords clash when sounded together and create a dissonant chord.
There are a number of ways to finger this chord on the guitar this inversion is up on the 8th fret:
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The C major chord uses the notes C, E and G
The F# major chord uses the notes F#, A# and C#
Polychords use the notes of two chords together and are written like this:
C
F#
Polychords shouldn't be confused with slash chords. Slash chords indicate a chord with an alternate bass note. So C / F# is a slash chord that is played as a C major chord with F# in the bass.