5.4 The iii (III) chord

5.4 The iii (III) chord

The iii chord is the diatonic triad built on scale degree $\hat{3}$: in a major key it is a minor triad, while in a minor key its quality is typically represented as a major triad, written $\mathrm{III}$. Its sound often connects tonic-oriented harmony with predominant or tonic-area motion, but its label depends on both scale degree and chord quality.

Finding and labeling the iii chord

In C major, the scale is

$$ \mathrm{C-D-E-F-G-A-B-C}. $$

The triad beginning on scale degree $\hat{3}$ uses $E$, $G$, and $B$:

$$ E-G-B. $$

The interval from $E$ to $G$ is a minor third, and the interval from $G$ to $B$ is a major third. Together they form a minor triad, so the Roman-numeral label is lowercase:

$$ \mathrm{iii}. $$

In A minor, the diatonic triad on scale degree $\hat{3}$ is built on $C$:

$$ C-E-G. $$

That triad is major, so it receives the uppercase label

$$ \mathrm{III}. $$

The uppercase/lowercase distinction is not cosmetic: it communicates chord quality.

Roman numerals versus lead-sheet symbols

Two labeling systems answer different questions. A Roman-numeral chord label identifies a chord’s scale-degree position within a key and uses case to show quality. A lead-sheet chord symbol gives performers the chord’s root and quality directly, usually without requiring them to infer the key.

Chord quality may be abbreviated with symbols such as $m$ for minor, $d$ for diminished, and $A$ for augmented. Thus, the C-minor triad may be labeled

$$ \mathrm{Cm}. $$

Lead-sheet chord symbols appear above the notated melody and indicate the specific chord progression expected for accompaniment. In C major, the iii chord can therefore be represented in two complementary ways:

System	Label	Information emphasized
Roman numeral	$\mathrm{iii}$	Scale-degree position and minor quality in the current key
Lead-sheet symbol	$\mathrm{Em}$	Root $E$ and minor quality

Misconception check — “iii” means the same thing as “Em.” The labels can refer to the same sounding chord in C major, but they are not interchangeable in every context. $\mathrm{iii}$ depends on the key; $\mathrm{Em}$ names the chord itself. If the key changes, the Roman numeral may change even when the sounding chord remains $E$ minor.

The iii chord in harmonic context

A chord’s identity is not enough; its neighbors matter. The iii chord can participate in a smooth bass motion, such as

$$ \mathrm{I-iii-IV}, $$

where the roots move upward by step:

$$ C \rightarrow E \rightarrow F. $$

It may also appear as part of a tonic-area prolongation, especially when the surrounding melody or bass makes its function clear. Do not automatically treat every chord containing scale degree $\hat{3}$ as $\mathrm{iii}$: determine the chord root, spell the triad, and then compare the root with the key.

Six-four chords near the iii chord

A six-four chord is a triad in second inversion, so its bass is the chord’s fifth. Not all six-four chords have the same function. The cadential six-four is especially important because it precedes the dominant:

$$ \mathrm{I^{6/4}-V-I}. $$

It commonly harmonizes the second note of a three-note ascending or descending figure. For example, a bass pattern

$$ G-A-G $$

may place the cadential six-four over $G$, the dominant over $A$, and the final tonic over $G$ in a suitable tonal context. The six-four sonority sounds like tonic, but its motion and placement make it a dominant embellishment rather than an independent tonic arrival.

By contrast, a neighboring or pedal six-four decorates a sustained or repeated bass note. The bass remains stable while the upper voices move away and return, so the six-four is heard as an embellishment of the surrounding harmony rather than as a cadential dominant preparation.

Writing and analysis warnings

Plagal cadences use the progression

$$ \mathrm{IV-I} $$

in major keys or

$$ \mathrm{iv-i} $$

in minor keys. They should not be confused with the cadential six-four, which leads into $\mathrm{V}$ before resolving to tonic.

In part-writing exercises, avoid poor successions such as

$$ \mathrm{V-IV} \qquad \text{and} \qquad \mathrm{V-vi}. $$

The latter may represent deceptive motion in an appropriate context, but it should not be inserted casually. Likewise, do not choose $\mathrm{iii}$ merely because its notes fit the melody: the progression must support a convincing harmonic function and acceptable voice leading.

Retrieval check: In G major, identify the notes and Roman numeral of the triad built on scale degree $\hat{3}$. Then give its lead-sheet symbol.
Answer: G major is $G-A-B-C-D-E-F^\sharp$; scale degree $\hat{3}$ is $B$, producing $B-D-F^\sharp$, a minor triad:

$$ \mathrm{iii}=\mathrm{Bm}. $$