What defines a diminished seventh chord?

A diminished seventh chord is characterized by its unique combination of intervals, specifically a diminished triad combined with a diminished seventh. The diminished triad consists of a root, a minor third, and a diminished fifth. When you add a diminished seventh to this triad, it creates a chord that is sonically tense and often resolves to more stable chords.

The diminished seventh is one semitone lower than a minor seventh, which further contributes to the dissonant quality of the chord. This configuration allows the diminished seventh chord to be used effectively in various harmonic contexts, as it can resolve in multiple directions, creating a rich and dynamic harmonic palette.

The other options describe different types of chords. For instance, a diminished triad plus a major seventh would yield a different type of harmonic sound that lacks the unique tension produced by the diminished seventh interval. A major triad with a minor seventh or a major seventh serves different functional roles and conveys different emotions in music, deviating from the distinctive characteristics of a diminished seventh chord.

Thus, the correct identification of a diminished seventh chord as a diminished triad along with a diminished seventh is fundamental for understanding its role in music theory and composition.