What is the quality of the seventh chord formed from a diminished triad?

The quality of the seventh chord formed from a diminished triad is characterized as a diminished 7th. This is established based on the intervals that make up the chord. A diminished triad consists of a root, a minor third, and a diminished fifth. When a seventh is added to this triad, it is a diminished seventh above the root note, resulting in a complete diminished seventh chord.

In musical theory, the diminished 7th chord is built by stacking two minor thirds on top of the diminished triad, creating a tense and dissonant sound. The diminished 7th chord is often notated as a diminished chord with the seventh note, reinforcing its identity as composed of only minor thirds, which further defines its sound quality.

This understanding contrasts with other chord qualities, such as half-diminished (which would include a major third on top of the triad), minor seventh (which involves a minor triad and a minor seventh), and dominant seventh (typically built on a major triad with a minor seventh). Thus, identifying the quality associated with the seventh chord built on a diminished triad directly leads to recognizing it as a diminished seventh chord.