The ii chord: the supertonic
The diatonic triad built from scale degree 2, called the ii chord, is a minor predominant chord. It consists of scale degrees 2, 4, and 6. In the same way that the dominant chord, based a fifth above the tonic, has a tendency to resolve to the tonic; the ii chord, based a fifth above the dominant, has a tendency to resolve to the dominant. It shares its root note with the dominant chord, and its scale degrees 4 and 6 naturally resolve to the dominant’s degrees 5 and 7.
The vi chord: the submediant
The minor diatonic triad built on scale degree 6, called the vi chord, is a minor chord including scale degrees 6, 1, and 3, and sharing two notes with both tonic and subdominant chords. It is sometimes classified as a substitute for, or an extension of, the tonic; however, it is most commonly considered to be a subdominant chord. Like the V and ii chords, it has a tendency to resolve down a fifth, in this case to the predominant ii chord.
The iii chord: the mediant
The minor diatonic triad built on scale degree 3, called the iii chord, consists of degrees 3, 5, and 7. Its function is somewhat ambiguous, as it shares two notes with both the tonic and dominant chords. Because of this, it is sometimes considered to be a substitution for both tonic and dominant chords. It is also sometimes considered a “weak” predominant, as it resolves to other predominant chords, especially down a fifth to the vi chord.
The vii° chord: the leading tone
The triad built on scale degree 7, called the vii° chord and consisting of scale degrees 7, 2, and 4, is the only diminished triad which occurs naturally in major keys. Its root, the leading tone, has a strong tendency to resolve up to the tonic note; and its scale degrees 2 and 4 naturally resolve to the tonic chord’s scale degrees 1, 3, and 5. Because of its role of resolving to the tonic, it is considered to be a dominant chord.