The iii (III) Chord - AP Music Theory Study Guide

Getting Started

Having established the foundational roles of tonic, predominant, and dominant harmonies, we now turn to the secondary diatonic triads. This chapter examines the mediant triad, built on the third scale degree. You will learn why this chord is used sparingly in the major mode and how its major-quality counterpart in the minor mode plays a unique and important role in harmonic progression.

What You Should Be Able to Do

• Identify the mediant triad by its Roman numeral (iii or III) in notated and performed music.

• Provide a correct Roman numeral analysis for progressions that include the mediant triad.

• Describe the typical harmonic function and common contexts for the iii chord in major keys.

• Explain the special function of the III chord in minor keys as it relates to the relative major.

• Differentiate between the usage and frequency of the mediant triad in major versus minor keys.

Key Concepts & Analysis

This section analyzes the mediant triad through the lens of Functional Harmony, which categorizes chords based on their role in creating or resolving musical tension within a key.

Functions & Allowable Motions

A harmonic function describes the role a chord plays in a progression, typically as a point of stability (Tonic), a transition away from stability (Predominant), or a point of high tension that demands resolution (Dominant).

The mediant triad is built on scale degree ^3. In a major key, the triad is minor and labeled iii. In a minor key, the triad is major and labeled III.

• The iii Chord in Major Keys: The iii chord has a weak and ambiguous function. It shares two pitches with the tonic I chord (scale degrees ^3 and ^5) and two pitches with the dominant V chord (scale degrees ^5 and ^7). This overlap prevents it from having a strong, independent identity. It is not a reliable member of any functional family and is best understood as a transitional or passing chord.

  • Allowable Motion: The most common progressions involving iii are I → iii → IV or I → iii → vi. In these contexts, it serves as a bridge connecting the tonic to a predominant or submediant chord. It very rarely moves directly to the dominant (V).

• The III Chord in Minor Keys: The III chord in a minor key has a much clearer and more significant function. It is the tonic triad of the relative major key.

  • Definition: The relative major is the major key that shares the same key signature as a given minor key. For example, the relative major of a minor is C major. The III chord in a minor (C-E-G) is the I chord in C major.

  • Allowable Motion: The primary role of the III chord is to signal a temporary or permanent shift to the relative major. A harmonic progression (a sequence of chords) like i → III often initiates this shift. The III chord then acts as a new tonic, followed by chords that confirm the relative major key (e.g., IV or V in the new key).

Cadence Implications

The mediant triad is almost never used in cadences. Its functional weakness in major and its role as a key-shifting device in minor make it unsuitable for creating the conclusive (or inconclusive) gestures that define cadences in the common-practice style.

Data & Organization Tools

This function map illustrates the limited role of the mediant within the standard T–PD–D–T framework. The most common paths are shown with solid arrows.

Harmonic Function Map

• Tonic (T)

  • I / i

  • vi / VI

• Predominant (PD)

  • IV / iv

  • ii / ii°

• Dominant (D)

  • V

  • vii°

Common Mediant Progressions:

• In Major: I → iii → vi

• In Major: I → iii → IV

• In Minor: i → III (functions as I in the relative major)

Evidence Bank

• Mediant Triad: The triad built on the third scale degree (^3) of a key. It is minor (iii) in major keys and major (III) in minor keys.

• Harmonic Progression: A purposeful sequence of chords that underlies the harmony of a piece of music.

• iii in Major: A minor triad used infrequently in 18th-century harmony, most often as a passing chord connecting I to vi or IV.

• III in Minor: A major triad that is functionally significant. It is the tonic of the relative major and is often used to prepare a modulation to that key.

• Relative Major: The major key built on ^3 of a minor key, sharing the same key signature. The III chord is its tonic.

• Functional Ambiguity: The characteristic of the iii chord in major, which shares notes with both tonic and dominant, weakening its functional identity and restricting its use.

Skill Snapshots

Functional

• Progression:I → iii → IV → V → I

  • Why it works: The iii chord functions as a smooth harmonic bridge between the tonic (I) and the predominant (IV).

• Progression:I → V6 → I → iii → vi

  • Why it works: After establishing the tonic, the iii chord gently moves the harmony away from I toward the submediant (vi), extending the tonic area.

• Progression (in a minor):i → V → i → III → V7/III → III

  • Why it works: After confirming the minor tonic (i), the progression moves to III, which is then treated as a new tonic (the key of C major), confirmed by its own dominant seventh chord.

Voice-Leading

• Rule: When moving from I to iii, keep the two common tones in the same voices.

  • Effect: This creates smooth, stepwise motion in the remaining voice, emphasizing the chord's role as a gentle continuation of the tonic harmony.

• Rule: The iii chord in major contains the leading tone (^7).

  • Effect: This note's strong tendency to resolve upward to ^1 is a primary reason why iii moves most naturally to vi or IV, where this resolution can be accomplished smoothly.

• Rule: The III chord in minor uses the natural form of scale degree 7 (♭^7).

  • Effect: The absence of the raised leading tone weakens the pull back to the minor tonic (i), making the move to the relative major sound more convincing.

Form

• Baseline Phrase: A phrase in C major ending with the progression IV–V–I firmly establishes the home key.

• Contrast: A subsequent phrase in a minor that moves quickly to a III chord and cadences in C major creates a clear formal and tonal contrast.

• Continuity: Within a phrase in C major, the progression I–iii–vi extends the opening tonic harmony without creating a strong sense of arrival, thus maintaining forward momentum.

Common Misconceptions & Clarifications

• Misconception: The iii chord is a dominant-function chord because it contains the leading tone.

  • Clarification: While it contains ^7, its overall sound and lack of other dominant-tendency notes (like ^2) give it a much weaker pull to tonic. Its function is not dominant.

• Misconception: The iii and III chords are functionally the same.

  • Clarification: They have fundamentally different roles. The iii in major is a rare passing chord; the III in minor is a common and structurally important chord that points to the relative major.

• Misconception: The iii chord can precede any other chord.

  • Clarification: Its usage is highly restricted in 18th-century style. It almost never moves directly to V and is not found at cadences. Its primary destinations are vi and IV.

Summary

The mediant triad, built on the third scale degree, has a dual identity in common-practice harmony. In major keys, the minor iii chord is one of the least frequently used diatonic triads. Its functional ambiguity, sharing tones with both tonic and dominant, relegates it to a secondary role as a passing chord, most often connecting I to vi or IV. In minor keys, however, the major III chord is far more common and functionally significant. It serves as the tonic of the relative major key, acting as a gateway to modulation or tonicization. A proficient analysis of harmonic progressions requires recognizing the stark difference in function between the mediant triad in its major- and minor-key contexts.