Why is the triad built on the seventh scale degree unique?

Why is the triad built on the seventh scale degree unique?

• A. It consists of two major thirds
• B. It contains a flat seventh
• C. It consists of two minor thirds
• D. It forms a perfect fifth with the root

Answer: (C)

The key idea is that the triad built on the seventh scale degree is diminished because it’s formed by stacking two minor thirds. Take the seventh degree in a major key as an example: in C major, that note is B, so the triad on it would be B–D–F. Each step from root to its third and from the third to the fifth is a minor third, giving two consecutive minor thirds. This interval pattern makes the triad diminished (the root-to-fifth interval is a diminished fifth, not a perfect fifth). That’s what makes it unique. The other statements don’t fit this interval structure: two major thirds would create a major triad, and a perfect fifth isn’t what appears from stacking those thirds on the seventh degree. The idea of a flat seventh doesn’t describe the triad’s built intervals either.

The key idea is that the triad built on the seventh scale degree is diminished because it’s formed by stacking two minor thirds. Take the seventh degree in a major key as an example: in C major, that note is B, so the triad on it would be B–D–F. Each step from root to its third and from the third to the fifth is a minor third, giving two consecutive minor thirds. This interval pattern makes the triad diminished (the root-to-fifth interval is a diminished fifth, not a perfect fifth). That’s what makes it unique. The other statements don’t fit this interval structure: two major thirds would create a major triad, and a perfect fifth isn’t what appears from stacking those thirds on the seventh degree. The idea of a flat seventh doesn’t describe the triad’s built intervals either.