---
title: "Major Third — AP Music Theory Definition & Exam Guide"
description: "A major third is an interval spanning four half steps, the bright-sounding building block of major chords. On AP exams, know it inverts to a minor sixth."
canonical: "https://fiveable.me/ap-music-theory/key-terms/major-third"
type: "key-term"
subject: "AP Music Theory"
unit: "Unit 2"
---

# Major Third — AP Music Theory Definition & Exam Guide

## Definition

A major third (M3) is an interval spanning four half steps (two whole steps), like C to E. It gives major chords their bright sound, and it inverts to a minor sixth because major intervals become minor when inverted and interval sizes sum to nine (3 + 6 = 9).

## What It Is

A major third is the distance between two pitches that are four [half steps](/ap-music-theory/key-terms/half-step "fv-autolink") (semitones) apart, like C up to E or G up to B. Count letter names and you get a [third](/ap-music-theory/unit-3/triad-chord-qualities-m-m-d-a/study-guide/C2Wj35yXuDEj0tYdyQcc "fv-autolink") (C-D-E is three letters); count half steps and you get four, which makes it *major* rather than minor. It's the interval that gives major triads their characteristically bright, stable color, since every major chord is built from a major third on the bottom and a minor third on top.

For [Topic 2.6](/ap-music-theory/unit-2/interval-inversion-compound-intervals/study-guide/fSSC2N938I34CjcsI5m5 "fv-autolink"), the part that matters is what happens when you flip it. Invert a major third by moving the lower note up an octave (C-E becomes E-C) and you get a minor sixth. This follows the two rules in PIT-1.M.1 every time: quality flips (major becomes minor) and the sizes of an interval and its inversion always add up to nine (3 + 6 = 9). Think of the octave as a pie. The major third and the minor sixth are the two slices that together make one whole octave, which is why they're inversions of each other.

## Why It Matters

The major third lives in [Unit 2](/ap-music-theory/unit-2 "fv-autolink") (Music Fundamentals II) under Topic 2.6, Interval Inversion and Compound Intervals. Learning objective [AP Music Theory](/ap-music-theory "fv-autolink") 2.6.A asks you to identify interval inversions and compound intervals in both performed and notated music, and the major third is one of the most common intervals you'll be asked to invert. Per PIT-1.M.1, an interval plus its inversion equals a perfect octave, so when an octave is split into two smaller intervals, those two intervals are inversions of each other. The major third / minor sixth pair is the textbook example of the 'major becomes minor' rule. Beyond Topic 2.6, the major third is the DNA of major triads, so getting fast and accurate with it pays off in every harmony, chord-spelling, and ear-training task that comes later in the course.

## Connections

### [Minor Third (Unit 2)](/ap-music-theory/key-terms/minor-third)

The [minor third](/ap-music-theory/key-terms/minor-third "fv-autolink") is one half step smaller (three semitones instead of four), and it's the other half of triad construction. A major triad stacks major third then minor third; a minor triad reverses the order. That one half step is the entire difference between a 'happy' chord and a 'sad' one.

### [Perfect Fifth (Unit 2)](/ap-music-theory/key-terms/perfect-fifth)

Stack a major third (C-E) plus a minor third (E-G) and you span a [perfect fifth](/ap-music-theory/key-terms/perfect-fifth "fv-autolink") (C-G). The fifth is also the classic inversion-rule contrast case, since perfect intervals stay perfect when inverted (a P5 flips to a P4), while the major third changes quality to a minor sixth.

### [Chord Progression (Units 3-4)](/ap-music-theory/key-terms/chord-progression)

Every [major chord](/ap-music-theory/key-terms/major-chord "fv-autolink") in a progression is built on a major third above its root. When you analyze or spell chords later in the course, identifying the third above the root is how you tell major harmonies from minor ones at a glance.

### [Dissonance (Unit 2)](/ap-music-theory/key-terms/dissonance)

The major third sits firmly on the consonant side of the line. That stability is why thirds (and their inversions, sixths) are the workhorse intervals of two-voice writing, while seconds and sevenths create the tension that needs resolving.

## On the AP Exam

The major third shows up in two main ways. First, straight interval identification: you see or hear two notes and have to name the interval, where the trap is confusing a major third (four half steps) with a minor third (three half steps). Second, inversion questions in the style of 'What is the inversion of a major third?' The answer is a minor sixth, and you get there with two quick checks: sizes sum to nine (3 + 6 = 9) and major flips to minor. Multiple-choice questions regularly cycle through the inversion pairs (P5 to P4, M3 to m6, M6 to m3, m3 to M6), so memorize the system rather than individual answers. Aural questions under 2.6.A can also play the interval, so practice hearing the major third's bright, open quality (it's the start of 'Oh, When the Saints').

## Major Third vs Minor Third

Both are thirds (three letter names apart), but the major third spans four half steps while the minor third spans only three. C to E is major; C to Eb is minor. They also invert differently: a major third becomes a minor sixth, while a minor third becomes a major sixth. The size always sums to nine in both cases, but the quality flips in opposite directions. If you're unsure which third you're looking at, count the half steps; four means major, three means minor.

## Key Takeaways

- A major third spans four half steps, or two whole steps, like C up to E.
- The inversion of a major third is a minor sixth, because major intervals become minor when inverted and interval sizes always add up to nine.
- An interval plus its inversion equals a perfect octave, so the major third and minor sixth together fill exactly one octave (PIT-1.M.1).
- To invert any interval, move the lower note up an octave and rename what's left.
- The major third is the bottom interval of every major triad, which is what gives major chords their bright sound.
- A major third differs from a minor third by exactly one half step, and that single half step changes both the interval's quality and what it inverts to.

## FAQs

### What is a major third in music theory?

A major third is an interval spanning four half steps (two whole steps) between notes three letter names apart, like C to E or F to A. It's the interval that defines major chords and is a core consonant interval in AP Music Theory Unit 2.

### What is the inversion of a major third?

A minor sixth. Interval sizes always sum to nine (3 + 6 = 9), and major intervals become minor when inverted. So C-E (major third) flipped becomes E-C (minor sixth).

### How is a major third different from a minor third?

A major third spans four half steps (C to E) while a minor third spans three (C to Eb). They also invert to different intervals: a major third becomes a minor sixth, but a minor third becomes a major sixth.

### Does a major third stay major when you invert it?

No. Only perfect intervals keep their quality when inverted. A major third becomes a minor sixth, since major and minor swap on inversion, just like diminished and augmented swap.

### Is C to E always a major third?

C natural to E natural is, yes, because it spans four half steps. But accidentals change the quality: C to Eb is a minor third (three half steps), and C# to E is also a minor third. Always count half steps, not just letter names.

## Related Study Guides

- [2.6 Interval Inversion and Compound Intervals](/ap-music-theory/unit-2/interval-inversion-compound-intervals/study-guide/fSSC2N938I34CjcsI5m5)

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