The Diminished Triad Sound
Two stacked minor 3rds — and 3 near misses, each a single tone away.
The chord-quality drill loads here.
Built upward, the diminished triad is two stacked minor 3rds — 3 tones in all. In a drill it arrives in a key you did not choose, so the only thing left to listen to is that shape.
Near missesminor triad, diminished 7th and half-diminished 7th — each one is this chord with a single tone moved, which is why a wrong answer here is almost never random.
It sits in the edge tier — the shapes whose difficulty is that they refuse to name a root.