The dominant and subdominant anon are more stable than the submediant and the supertonic.
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The most common practice, for both Beethoven and other composers, the mediant or submediant, rather than the dominant, is used for the second group.
From the en.wikipedia.org
For example, if the Aeolian mode is used in its all-white-note pitch based on A, this would be an A-minor triad, which would be the submediant in the relative major key of C major.
From the en.wikipedia.org
More examples
(music) the sixth note of a major or minor scale (or the third below the tonic)