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three-note chord (apex)
Three Chord Opera was created in 2000.
Three Chord Revolution was created in 2003.
An accidental chord is a chord which contains one or more notes which are considered foreign to the key in which the song is written.An example might be to include an E flat major chord in a song played in C major.
A three-note chord must be comprised of three different notes, and is also known as a triad.