In our previous lesson, we looked at the foundation of chords. In this lesson, we will move a step further, treating the Diminished Chords

Diminished Triad Piano Chord
Chord Type:​ Diminished triad
Formula:​ 1+♭3+♭5

Do you see a pattern here? Most chords will use some version of the 1, 3, and 5th tones (later, we’ll add in the 7th tone, which is HUGE in music). Diminished chords take the sad and serious sound of
minor chords even further by flatting yet another tone (the 5th). This gives you a scary­sounding chord, but is used in tons of situations.

Notes in C:​ C + Eb + Gb ................... http://musicalcrib.com/piano-chords2-diminished-and-augmented-chords/