A diminished seventh chord is a type of chord made up of four notes, each a minor third apart. It is typically represented by the 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. Diminished seventh chords are commonly found in various musical genres, including classical and jazz. They can function as leading tones to resolve to other chords, making them useful for transitions. The chord can be transposed to different keys, maintaining its structure and sound quality.