A diminished seventh chord is a four-note chord formed by stacking minor thirds. It consists of a root note, a minor third, a diminished fifth, and a diminished seventh. This chord has a tense and unstable sound, often used to create tension in music. In music theory, the diminished seventh chord can be used in various keys and is often found in classical, jazz, and popular music. It can resolve to other chords, providing a sense of release and movement. Its symmetrical structure allows for easy transposition, making it versatile for composers and musicians.