Diatonic Chords and Roman Numerals - AP Music Theory Study Guide

Getting Started

Having learned to identify the notes within major and minor scales, we now move to the next level of musical structure: harmony. This chapter introduces diatonic chords—the fundamental harmonic building blocks derived directly from a key’s scale. We will learn a powerful analytical system using Roman numerals to name these chords, understand their quality, and, most importantly, uncover their specific function within a musical phrase.

What You Should Be Able to Do

• Identify the seven diatonic triads and seventh chords in any major or minor key.

• Provide a complete Roman numeral analysis for a given harmonic progression in notated or performed music.

• Determine the quality (major, minor, diminished) of a diatonic chord based on its scale degree root.

• Label chords in root position and inversion using correct Roman and Arabic numerals.

• Explain the harmonic function (tonic, predominant, or dominant) of any diatonic chord.

Key Concepts & Analysis

The foundation of harmonic analysis is understanding that not all chords are created equal; each has a specific role or job to do within the context of a key. This concept is called harmonic function. By labeling chords with Roman numerals, we can move beyond simply naming them (e.g., "C major triad") to describing their role in the larger harmonic story (e.g., "the tonic chord in C major").

A diatonic chord is a triad or seventh chord built using only the notes of a given major or minor scale. A Roman numeral analysis is a method of labeling these chords. The numeral itself indicates the scale degree of the chord's root, while its case and other symbols indicate the chord's quality.

• Uppercase numerals (I, IV, V) denote major triads.

• Lowercase numerals (ii, iii, vi) denote minor triads.

• A degree symbol (vii°) denotes a diminished triad.

Functions & Allowable Motions

In tonal music, chords are typically grouped into one of three functional categories that create a cycle of tension and release. The progression of these functions gives music its sense of forward momentum and logical structure.

• Tonic (T) Function: Represents stability and rest. This is the harmonic "home base."

  • Primary Chords: I in major, i in minor.

  • Common Motion: The tonic chord can move to any other chord. It is the beginning and end point of most harmonic progressions.

• Predominant (PD) Function: Represents transition and preparation. These chords create moderate tension by moving away from the tonic and setting up the arrival of the dominant.

  • Primary Chords: IV and ii in major, iv and ii° in minor.

  • Common Motion: Predominant chords progress most naturally to dominant-function chords.

• Dominant (D) Function: Represents the highest level of harmonic tension. These chords have a strong gravitational pull back to the tonic, creating a powerful need for resolution.

  • Primary Chords: V and vii° in major and minor. (Note: In minor keys, the V chord is made major by raising the leading tone).

  • Common Motion: Dominant chords progress almost exclusively to tonic-function chords.

The most common and logical path through these functions is Tonic → Predominant → Dominant → Tonic. While variations exist, this fundamental progression (T → PD → D → T) forms the backbone of Western tonal music. Moving backward in this cycle, such as from D to PD, is called a harmonic retrogression and is generally avoided as it weakens the progression's forward drive.

Common Progressions

This functional grammar gives rise to a set of highly common and effective chord progressions:

• The foundational progression: I–V–I or i–V–i

• The complete functional cycle: I–IV–V–I or i–iv–V–i

• Using the supertonic as predominant: I–ii–V–I or i–ii°–V–i

• Expanding the tonic area: I–vi–IV–V–I

Cadence Implications

The end of a musical phrase, known as a cadence, is defined entirely by the interaction of these harmonic functions. An authentic cadence, which provides a sense of finality, is created by a D → T progression (V–I or V–i). A half cadence, which creates a sense of pausing or expectation, is created when a phrase ends on a dominant-function chord (any chord moving to V).

Data & Organization Tools

The table below outlines all the diatonic triads for major and harmonic minor keys, along with their standard Roman numeral and typical harmonic function. The harmonic minor scale (with its raised ^7) is the standard reference for building functional chords in minor keys, as it provides the crucial leading tone needed for a strong dominant chord.

Evidence Bank

• Diatonic Triads: The seven basic three-note chords built from the notes of a key's scale, forming the primary palette for tonal harmony. They are identified by Roman numerals I through vii°.

• Diatonic Seventh Chords: Four-note chords built from the notes of a key's scale (e.g., Imaj7, V7, iiø7). The V7 chord is especially important for its strong dominant function.

• Roman Numeral: The core analytical symbol indicating the scale-degree root and quality of a chord. For example, IV in C major is an F major triad.

• Arabic Numerals (Figured Bass): Numbers written after a Roman numeral to indicate inversion. For example, 6 indicates first inversion (I6) and 6/4 indicates second inversion (I6/4).

• Tonic Function: The harmonic role of providing stability and resolution. The I (or i) chord is the ultimate goal of harmonic motion.

• Dominant Function: The harmonic role of creating tension that demands resolution to the tonic. This function is fulfilled by chords built on ^5 (V) and ^7 (vii°).

• Predominant Function: The harmonic role of preparing the arrival of the dominant. This function is fulfilled by chords built on ^4 (IV, iv) and ^2 (ii, ii°).

• Leading-Tone Chord (vii°): The diminished triad built on the seventh scale degree. It shares two notes with the V7 chord and has a strong dominant function.

Skill Snapshots

Functional

• I – IV – V – I: This progression is a perfect illustration of the T → PD → D → T cycle, moving from home base, to preparation, to maximum tension, and back to resolution.

• i – ii°6/5 – V7 – i: A common minor-key progression showing how an inverted predominant seventh chord (the ii°6/5) smoothly connects the tonic to the dominant.

• I – vi – ii – V: This progression first expands the tonic area with the submediant chord (vi, which shares notes with I) before moving through the PD → D part of the functional cycle.

Voice-Leading

• Rule: The leading tone (^7) in a V or vii° chord is a powerful tendency tone. Effect: Its resolution upward by half step to the tonic note (^1) is the primary driver of the dominant-to-tonic relationship.

• Rule: The seventh of a dominant seventh chord (V7) is a dissonant tendency tone. Effect: It resolves downward by step, creating smooth voice-leading and resolving the tritone inherent in the chord.

• Rule: Avoid harmonic retrogression, such as V–IV. Effect: Following the standard functional path (PD → D) creates a sense of logical forward motion; reversing it weakens this drive.

Form

• Baseline Phrase: A musical phrase ending with a V–I progression concludes with an authentic cadence, creating a strong sense of closure.

• Contrast: A phrase ending on a V chord creates a half cadence. This lack of resolution leaves the listener expecting more, making it an effective way to connect two phrases.

• Continuity: A phrase ending with a V–vi progression creates a deceptive cadence. This thwarts the listener's expectation for tonic, providing a surprise while still resolving the dominant's tension, often to extend the phrase.

Common Misconceptions & Clarifications

• Uppercase vs. Lowercase: The case of a Roman numeral always indicates the chord's quality (major or minor), not its function or importance. A IV is major; a iv is minor.

• Minor Key Chords: The dominant (V) and leading-tone (vii°) chords in minor keys are almost always built using the raised ^7 from the harmonic minor scale. This makes the V chord major and the vii° chord diminished, creating the essential leading-tone-to-tonic pull that defines dominant function.

• Function vs. Scale Degree: A chord's Roman numeral is its name (e.g., ii); its function is its job (e.g., predominant). Multiple chords can have the same function (e.g., both ii and IV are predominants).

• Inversions and Function: Inverting a chord does not change its fundamental harmonic function. A I6 chord is still tonic, and a V4/2 is still dominant. Inversion primarily affects the chord's stability and the melodic line of the bass.

Summary

Diatonic chords are the harmonic foundation of tonal music, built directly from the notes of a scale. We use a system of Roman and Arabic numerals to analyze these chords, identifying the scale degree of the root, the chord's quality, and its inversion. More importantly, these chords are organized by a clear grammar of harmonic function: tonic chords provide stability, predominant chords prepare for tension, and dominant chords create tension that resolves back to the tonic. This T–PD–D–T cycle creates logical and compelling progressions that give music its sense of direction and coherence. Mastering the identification and function of these chords is the first major step toward a deep understanding of how harmony works.