What is the function of the iii chord, and where does it fit in a progression?
Topic 5.4 The iii (III) Chord: use the mediant as a connecting chord between I and IV (or vi) and part-write it within a diatonic progression.
A focused answer to AP Music Theory Topic 5.4, covering the mediant iii (III in minor), its weak and ambiguous function, its common use to harmonise a descending scale degree 7 to 6 in the soprano or to connect I to vi or IV, and smooth part-writing, with a worked progression.
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What this topic is asking
The College Board (Topic 5.4) wants you to understand the mediant iii (III in minor) as a weak, connecting chord, recognize its common use to harmonise a descending soprano from scale degree 7 to 6 or to link I to IV or vi, and part-write it smoothly within a diatonic progression.
A weak, ambiguous chord
Because iii sits between the tonic and dominant in pitch content, it does not assert a clear function. This is why composers use it sparingly and mostly for connection rather than for harmonic weight.
The connecting role
In this role iii fills the gap between stronger chords, letting the soprano descend by step with each note harmonised. It is a melodic tool more than a functional one.
Why iii is used so little
The central idea is that a chord's usefulness depends on having a clear function, and iii does not. Tonic chords rest, predominants lead to the dominant, and dominants drive to the tonic, but the mediant straddles tonic and dominant content without committing to either, so it cannot do any of those jobs strongly. What it can do is connect: when the soprano needs to descend stepwise from the leading tone down to scale degree 6, iii provides a chord that harmonises the passing degree 7 smoothly. Recognizing this explains both why you rarely see iii and why, when you do, it is almost always supporting a descending line. In analysis, labelling a iii correctly and noting its connective role shows you understand function, not just chord spelling.
Part-writing the iii chord
To use iii, place it between two stronger chords (often I and IV or I and vi), support a descending soprano with it, and connect by keeping common tones and moving the other voices by step, checking for parallels.
Try this
Q1. Why is the mediant chord described as having an ambiguous function? [1 point]
- Cue. It shares two tones each with the tonic and the dominant, so it commits to neither function.
Q2. In what melodic situation is iii most commonly used? [2 points]
- Cue. To harmonise a soprano descending from scale degree 7 to 6, connecting I to IV or vi as a passing chord.
Exam-style practice questions
Practice questions written in the style of College Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AP 2022 (style)1 marksSection I (multiple choice, written). The mediant chord (iii) in a major key is best described as which of the following? (A) a strong dominant-function chord (B) a weak, connecting chord used sparingly (C) the most common predominant (D) a substitute for the tonic at cadencesShow worked answer →
The correct answer is (B) a weak, connecting chord used sparingly.
The mediant (iii) is the least used diatonic triad in major. It has an ambiguous function (it shares tones with both I and V) and is mostly used as a passing or connecting chord, often to harmonise a descending soprano line from scale degree 7 to 6.
(A) the strong dominant is V (and vii diminished). (C) the most common predominants are IV and ii. (D) the tonic substitute at a deceptive cadence is vi, not iii. The trap is overusing iii; it appears far less often than the other diatonic chords and rarely carries a cadence.
AP 2023 (style)2 marksSection II (free response, harmonic analysis). In a passage the soprano descends scale degree 1, 7, 6 over the chords I, iii, IV. Explain the function of the iii chord here.Show worked answer →
A 2-point analysis question.
(1 point) The iii chord harmonises the passing scale degree 7 in the soprano as the line descends from degree 1 to degree 6, connecting the tonic (I) to the subdominant (IV).
(1 point) Its function is connective and passing, not cadential; iii supports the descending melody and smooths the move from I to IV, rather than acting as a predominant or dominant in its own right.
Markers reward identifying iii as a passing or connecting chord that harmonises the descending soprano (degree 7), and noting that it is not functioning as a strong predominant or dominant.
Related dot points
- Topic 5.1 Adding Predominant Function IV (iv) and ii (ii diminished): use the subdominant and supertonic chords to prepare the dominant and part-write them smoothly.
A focused answer to AP Music Theory Topic 5.1, covering predominant function, the IV (iv) and ii (ii diminished) chords, how they lead to the dominant, the use of ii6 and first-inversion predominants, and smooth part-writing, with a worked predominant progression.
- Topic 5.2 The vi (VI) Chord: use the submediant as a tonic substitute and as the goal of a deceptive cadence, and part-write V to vi correctly.
A focused answer to AP Music Theory Topic 5.2, covering the submediant vi (VI) as a tonic substitute, its role in the deceptive cadence V to vi, the special doubling needed to avoid parallels in V to vi, and its use to extend a phrase, with a worked deceptive resolution.
- Topic 4.3 Harmonic Progression, Functional Harmony, and Cadences: explain tonic, predominant and dominant function, the normal direction of progressions, and the four cadence types.
A focused answer to AP Music Theory Topic 4.3, covering functional harmony (tonic, predominant, dominant), the normal flow tonic to predominant to dominant to tonic, and the four cadences (perfect authentic, imperfect authentic, half, plagal, with the deceptive cadence), with a worked cadence analysis.
- Topic 3.5 Roman Numerals and SATB: label diatonic chords with Roman numerals showing root and quality, and arrange chord tones in the SATB four-voice texture.
A focused answer to AP Music Theory Topic 3.5, covering Roman numeral analysis (case shows quality, figures show inversion), the diatonic numerals of major and minor keys, the SATB four-voice layout and ranges, and how to spell a chord across four voices, with a worked analysis.
- Topic 4.2 SATB Voice Leading: apply the rules of range, spacing, doubling, smooth motion and tendency-tone resolution when writing four-part harmony.
A focused answer to AP Music Theory Topic 4.2, covering the four-voice ranges, the spacing rule (no more than an octave between adjacent upper voices), doubling guidelines, the ban on parallels and voice crossing, and resolving the leading tone and tendency tones, with a worked voicing.
Sources & how we know this
- AP Music Theory Course and Exam Description — College Board (2024)