Diminished Triads On The Piano (Video) PlayPiano Posted on September 12, 2009 by DuaneNovember 25, 2020 A diminished triad is formed by lowering both the 3rd and the 5th of a major triad. For more complete information, please go to: https://www.playpiano.com/101-tips/6-diminished.htm after you watch this short video.