The most stable-sounding diatonic triad is typically considered to be the tonic triad, which is built on the first scale degree of a major or minor key. For example, in the key of C major, the tonic triad consists of the notes C, E, and G. This triad provides a sense of resolution and stability to the musical phrase. Additionally, in minor keys, the dominant triad (built on the fifth scale degree) can also be considered stable and strong-sounding. In the key of C minor, the dominant triad consists of the notes G, B♭, and D.
