AP® Music Theory · Unit 5 · Topic 5.4
The iii (III) Chord
The iii chord is a minor triad built on the third scale degree that mostly works as a quiet passing chord between I and IV, not as a dominant substitute.
The iii chord shows up less than any other diatonic triad, and that's because it shares two of its three notes with I and two with V. In C major, iii is E-G-B, and I is C-E-G, V is G-B-D. A chord that overlaps so heavily with its neighbors has little identity of its own, so composers reach for it rarely, usually when they want a soft, ambiguous color rather than a strong push in any direction.
Its most common job is as a connector. In root position, the bass line I to iii moves from scale degree 1 up to scale degree 3, a third, and then iii to IV moves scale degree 3 up to scale degree 4, a step. That second motion is smooth, which is why iii often appears wedged between I and IV: it eases the bass toward the predominant chord instead of leaping straight there. Because of this role, iii is sometimes described as a weak tonic substitute or a bridge into predominant harmony rather than a harmony with its own strong function.
iii contains the leading tone, scale degree 7, which might suggest it could stand in for V. It can't. What gives V its pull toward I is the tritone between scale degree 7 and scale degree 4, and iii does not contain scale degree 4 at all. Without that tritone, iii has no built-in tension demanding resolution, so it never substitutes for V. In minor keys, before the leading tone is raised, III is a major triad built on the unaltered scale degree 3 and functions more like a relative-major tonic color than a predominant or dominant stand-in.
Key terms
- iii chord
- The diatonic triad built on the third scale degree, minor in major keys.
- leading tone
- Scale degree 7, a half step below tonic, which pulls strongly upward toward scale degree 1.
- tritone
- An interval of three whole steps, here formed between scale degree 7 and scale degree 4, that drives V toward I.
- predominant function
- The role of chords like ii and IV that prepare and lead into the dominant chord.
- passing harmony
- A chord inserted between two others mainly to smooth the voice leading rather than to assert its own strong function.
Watch it explained
- How it worksHow does iii usually work as a passing harmony wedged between I and IV?
- Worked exampleFind the notes of iii in F major and where it sits in a I, iii, IV, V progression.
- Common mistakeWhy isn't iii a good substitute for V even though it contains the leading tone?
- Going furtherWhat happens to the III chord in a minor key once the leading tone is raised?
Check yourself
End of topic.