A C diminished chord is a three-note chord consisting of the notes C, E♭, and F♯. It is formed by stacking two minor thirds on top of each other. This creates a tense and unstable sound, often used in music to lead into more stable chords. Diminished chords, like the C diminished chord, are commonly found in various music genres, including classical, jazz, and pop. They can add drama and tension to a piece, making them effective for transitions or climactic moments in a song.