The III chord as a Dominant? I Don’t Think So

In our blog “Understanding Tonic, Subdominant, and Dominant Chords,” we laid out the foundational roles of chords within functional harmony. We did not classify the iii chord as a dominant-function chord—and for good reason. However, I’ve come to learn that some theorists and musicians argue that iii can substitute for a dominant chord in certain contexts. I disagree, and I’ll explain why. Still, in the spirit of open discussion, I’ll also present the reasoning behind this claim and why it has gained traction among some musicians.

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What Is the III Chord?

In diatonic harmony, the iii chord is the triad built on the third degree of the major scale. In the key of C major, the iii chord is E minor (E, G, B). It is a minor chord and typically considered part of the tonic family, as it shares two notes (E and G) with the tonic chord (C major: C, E, G). The iii chord is often used in harmonic progressions for color, smooth voice leading, or as part of tonic prolongation.

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The Argument for III as a Dominant Substitute

Some musicians and theorists argue that the iii chord can act as a dominant or a dominant substitute because of its shared notes with the dominant chord (V). In C major:

• The dominant chord G major consists of G, B, D.
• The iii chord E minor consists of E, G, B.

The argument is that since G and B are shared between the two chords, iii can function as a softer or weaker version of the dominant chord. In particular, in jazz and pop reharmonization, iii is sometimes used in place of V to create a smoother harmonic transition.

For example:

• Instead of C → G → C (I → V → I), you might see C → Em → C (I → iii → I).

While this may create a pleasing progression, I contend that it does not make the iii chord a true dominant-function chord. Let’s explore why.

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Why III Is Not a Dominant

1. Lack of Leading Tone Tension

The defining feature of a dominant chord is its tension, which resolves to the tonic. This tension comes primarily from the leading tone (B in the key of C), which resolves upward to the tonic note (C). The dominant chord (G) also contains the F note in its seventh chord form (G7: G, B, D, F), which resolves downward to the E in the tonic chord.

The iii chord lacks the note F entirely. Without this crucial element, it fails to generate the same pull toward the tonic. Instead, the iii chord feels more like a passing or neighboring chord rather than a destination-driven dominant chord.

2. Weak Tonal Ambiguity

The iii chord doesn’t strongly imply movement to any particular chord. While it can act as a bridge to vi (as in C → Em → Am), it doesn’t have the sense of inevitability that a true dominant chord possesses. This makes it feel more static or introspective rather than directional.

3. Functional Harmony Perspective

In classical functional harmony, chords are categorized as tonic, subdominant, or dominant. The iii chord is firmly in the tonic camp because of its shared notes with the tonic triad. By contrast, dominant-function chords like V or vii° are defined by their ability to create tension and drive the progression forward. The iii chord simply doesn’t meet this criterion.

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When III Can Function as a Dominant

There is one notable exception: when the iii chord is altered to become a dominant seventh chord, it can take on a dominant function. In the key of C major, for example, turning E minor (E, G, B) into E7 (E, G#, B, D) creates a secondary dominant that resolves strongly to Am, the vi chord. This is because:

• G# acts as the leading tone to A.
• D resolves downward to C in Am.

This progression—C → E7 → Am (I → V/vi → vi)—is common in classical and popular music. However, this altered iii chord (E7) is no longer truly a iii chord; it’s a secondary dominant.

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Uses of III in Classical and Modern Music

Despite its weakness as a dominant, the iii chord does have legitimate uses in classical and modern music:

1. Tonic Prolongation

In classical music, the iii chord often serves as part of tonic prolongation. For example:

• C → Em → Am → F (I → iii → vi → IV).

This creates a smooth voice-leading progression while staying within the tonic area.

2. Passing Motion

The iii chord frequently appears in stepwise basslines. For example:

• C → Em → F (I → iii → IV): The bass moves from C to E to F.

3. Color and Contrast

In jazz and pop, the iii chord is often used for its minor quality, providing emotional depth and contrast within progressions.

• Example: C → Em → Am → G (I → iii → vi → V).

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Why the Debate Exists

The idea of iii as a dominant likely arises from modern reharmonization practices, where shared notes and substitutions blur traditional harmonic boundaries. Jazz and pop musicians often prioritize smooth transitions and interesting textures over strict functional roles, making iii a viable substitute for V in some cases. However, this is a substitution, not equivalence.

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Conclusion

While the iii chord shares some notes with the dominant chord and can occasionally substitute for it in modern styles, it lacks the essential tension and resolution that define a true dominant-function chord. At its core, the iii chord is tonic-adjacent, and its role is better understood as part of tonic prolongation or as a coloristic chord. Only when altered (e.g., E7 as V/vi) does it take on a dominant function.

So, while some may argue that iii can be a dominant, I respectfully disagree. Its true strength lies in its ability to add subtlety and motion within the tonic framework, not in leading us back to it.

For more on the foundational roles of tonic, subdominant, and dominant chords, check out our blog [“Understanding Tonic, Subdominant, and Dominant Chords”]. Happy harmonizing!