5.4 The iii (III) Chord

TLDR

The mediant chord (iii in major, III in minor) is the diatonic triad built on scale-degree 3, and it shows up rarely in 18th-century style. When it does appear, it usually expands or prolongs the tonic, and in a minor key, the III chord most often represents the relative major key. For the AP Music Theory exam, recognize it when you see it, but do not treat it as a go-to predominant.

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Why This Matters for the AP Music Theory Exam

This topic sharpens your harmonic analysis and Roman-numeral skills. You need to identify chord function in both performed and notated music, and the mediant is the chord students most often mislabel. Knowing that iii (III) is rare, and that III in minor frequently points toward the relative major, helps you avoid analysis errors during score reading, harmonic dictation, and part-writing tasks where Roman numerals and implied harmonies matter.

Key Takeaways

• The mediant triad is built on scale-degree 3: lowercase iii in major keys, uppercase III in minor keys.
• The mediant is rarely used in 18th-century (common-practice) progressions. Do not reach for it as a normal predominant.
• When it appears, it often prolongs or expands the tonic, especially near the start of a phrase (I-iii or i-III).
• In a minor key, III usually represents the relative major key, so it can signal tonicization or modulation.
• If you see iii6 moving to V, the chord may really be acting as a dominant with a non-chord tone, not a true mediant.
• Use Roman numerals to label function accurately, and double-check any progression that puts iii in an unexpected spot.

The Mediant Chord Basics

The mediant chord is the last diatonic triad to learn in this set. It is built on scale-degree 3, the pitch named the mediant.

• In a major key it is minor, so you write it as iii.
• In a minor key it is major, so you write it as III.

Because it shares two notes with both the tonic triad and the dominant triad, the mediant has a weak, ambiguous identity. That is a big reason it rarely drives 18th-century progressions. When it does appear, think tonic prolongation first, not strong predominant motion.

Prolonging or Expanding the Tonic

The most useful way to think about iii (III) is as a tonic expansion. You can write a simple I-iii-I or i-III-i in the tonic area of a phrase.

A smoother option keeps motion in the bass and avoids parking on a stable root-position tonic:

• Try I-iii-I6 so the line moves away from root-position tonic and sets up the predominant section more naturally.

A common longer pattern inserts a non-functional IV after the mediant:

• I-iii-IV-I6

Here the IV is "non-functional" in the sense that it is not meant to set up a strong IV-I plagal cadence. Writing IV-I6 keeps that cadence weak so the whole span still sounds like an expanded tonic.

Voice-Leading Example

A clean way to write a I-iii-IV-I tonic expansion is to put a descending stepwise line in the soprano. Starting from the doubled root of the tonic, a soprano line of 8-7-6-5 fits perfectly:

• 8 (scale-degree 1) = root of I
• 7 (scale-degree 7) = fifth of iii
• 6 (scale-degree 6) = third of IV
• 5 (scale-degree 5) = fifth of I

This stepwise descent keeps the ear anchored on tonic. The inner notes sound like passing motion over harmonies that still feel like home.

Voice-leading reminders for this pattern:

• Double the root in each chord, especially the weak mediant.
• In the IV chord, you have some leeway to double the third so you do not emphasize a strong subdominant pull.
• Write the iii and IV chords in root position; the tonic can appear in first inversion, and a closing I6 is common.

When iii6 Looks Like a Dominant

You will almost never see the mediant standing alone as a predominant. If you spot something like I-iii-V, recheck your analysis.

There is one situation that explains many of these cases. In first inversion, the mediant contains scale-degrees 5, 7, and 3. The dominant contains scale-degrees 5, 7, and 2. So a first-inversion mediant sounds a lot like a dominant with an added non-chord tone.

That means a iii6-V progression often is not a true mediant at all. It usually works as a dominant where scale-degree 3 resolves down to scale-degree 2.

In a minor key, this is also where a III+ (augmented) chord can show up. Normally you do not raise the leading tone when spelling III in minor, because that creates a harsh augmented triad. But in a III+6-V progression, the raised leading tone signals that the chord is functioning as a dominant.

There is no special AP notation for this. Just label what you see, such as iii6-V or III+6-V. If you are writing a contextual analysis, you can note that the chord has dominant function.

III in Minor and the Relative Major

In a minor key, the III chord is the tonic of the relative major. For example, in B minor, III is D major, the relative major of B minor.

This matters because composers often move to other keys for variety:

• Tonicization borrows a few chords from another key for a brief moment.
• Modulation stays in the new key long enough that you hear it as the new "home" key.

Music in minor frequently modulates to the relative major. If you are analyzing a minor-key passage and suddenly see lots of III, VII, and VI chords, you may be in the relative major. In that reading:

• III acts as the major tonic
• VII acts as the major dominant
• VI acts as the major subdominant

Worked Reading

Suppose you are in D minor. The tonic is a D minor triad, and the dominant is A major with a raised C♯ leading tone. Then you start seeing many C naturals, plus F major, C major, and B♭ major chords.

Check the notes instead of forcing odd labels:

• A "VII-III" in D minor is just C-F, which is V-I in F major.
• A "B♭-C" move is IV-V in F major.

That pattern strongly suggests the music has modulated to F major, the relative major. Analyze the rest of that section in F major until the music transitions back to D minor.

How to Use This on the AP Music Theory Exam

Harmonic Analysis

• When you find scale-degree 3 in the bass moving to V, ask whether the chord is really a dominant with a non-chord tone before labeling it iii.
• In a minor key, a cluster of III, VII, and VI chords is a clue to check for a move to the relative major.
• Confirm chord function with Roman numerals rather than the chord's name alone.

Part Writing

• Use the mediant to expand the tonic, not to replace a strong predominant.
• For I-iii-IV-I6, write a descending stepwise soprano (such as 8-7-6-5) and double roots, with optional third-doubling in IV.
• Keep iii and IV in root position and consider ending the span on I6 for smoother motion.

Common Trap

• Do not insert iii as a normal predominant before V. If your progression reads I-iii-V, you have probably misanalyzed a dominant-function chord.

Common Misconceptions

• "The mediant is a normal predominant option." It is rare in 18th-century style and is weak at best as a predominant. Treat it mainly as tonic prolongation.
• "iii always functions as a mediant." A iii6 leading to V usually behaves like a dominant with a non-chord tone, where scale-degree 3 falls to scale-degree 2.
• "III in minor is just an unusual chord." In minor, III most often represents the relative major and can signal tonicization or modulation.
• "If iii and vi both appear, order does not matter." When both show up, iii almost always comes before vi, since vi is the stronger predominant.
• "Raising the leading tone in III is a mistake." A III+ usually appears specifically when the chord is acting as a dominant, which is why the leading tone is raised.

Related AP Music Theory Guides

• 5.1 Adding Predominant Function IV (iv) and ii (ii0) to a Melodic Phrase
• 5.6 Cadential 6/4 Chords
• 5.5 Cadences and Predominant Function
• 5.7 Additional 6/4 chords
• 5.3 Predominant Seventh Chords
• 5.2 The vi (VI) Chord