---
title: "Perfect Fourth — AP Music Theory Definition & Exam Guide"
description: "A perfect fourth spans four letter names and five half steps. It's the inversion of the perfect fifth, a relationship AP Music Theory tests directly in Unit 2."
canonical: "https://fiveable.me/ap-music-theory/key-terms/perfect-fourth"
type: "key-term"
subject: "AP Music Theory"
unit: "Unit 2"
---

# Perfect Fourth — AP Music Theory Definition & Exam Guide

## Definition

A perfect fourth is an interval spanning four letter names and five half steps (like C up to F). In AP Music Theory, it's best known as the inversion of the perfect fifth, since perfect intervals stay perfect when inverted and the two interval sizes add up to nine (4 + 5).

## What It Is

A perfect fourth is the [interval](/ap-music-theory/unit-2/interval-size-quality/study-guide/HxrxB0vETN0eDp83zij1 "fv-autolink") between two notes that are four letter names apart and exactly five [half steps](/ap-music-theory/key-terms/half-step "fv-autolink") apart. C up to F, G up to C, D up to G. The "perfect" label means it belongs to the family of intervals (unisons, fourths, fifths, octaves) that come in perfect, diminished, or augmented qualities rather than major and minor.

The AP exam cares most about the perfect fourth's relationship to the perfect fifth. Take any perfect fifth, move the bottom note up an octave, and you get a perfect fourth. That's [interval inversion](/ap-music-theory/unit-2/interval-inversion-compound-intervals/study-guide/fSSC2N938I34CjcsI5m5 "fv-autolink"), and per the CED (PIT-1.M.1), perfect intervals remain perfect when inverted, while the sizes of an interval and its inversion always sum to nine. So 5th + 4th = 9, and the quality stays perfect on both sides. You can also think of it as splitting an octave in two: a perfect fifth plus a perfect fourth fills the octave exactly.

## Why It Matters

The perfect fourth lives in **Topic 2.6 (Interval Inversion and Compound Intervals)** in **[Unit 2](/ap-music-theory/unit-2 "fv-autolink"): Music Fundamentals II**, supporting learning objective **2.6.A**, which asks you to identify interval inversions in both performed and notated music. That "performed" part matters. On the aural portion of the exam, you have to hear a fourth and distinguish it from a fifth, which is harder than it sounds because they're inversions of each other and share notes. On the written side, the perfect fourth is your fastest shortcut for spelling and checking fifths. If you know C to F is a perfect fourth, you instantly know F to C is a [perfect fifth](/ap-music-theory/key-terms/perfect-fifth "fv-autolink"), no half-step counting required. That inversion fluency pays off again when you start spelling triads and analyzing voice leading later in the course.

## Connections

### Interval (Units 1-2)

The perfect fourth is one specific interval, but it's the one that makes the [inversion](/ap-music-theory/unit-7/tonicization-through-secondary-dominant-chords/study-guide/rLCBduMA9EmMPYSSNiwd "fv-autolink") rules click. Once you see that a P4 and a P5 are the same two pitch classes flipped, the whole "sizes sum to nine, perfect stays perfect" system stops feeling like memorization.

### Interval Inversion and Compound Intervals (Unit 2)

Topic 2.6 is the perfect fourth's home turf. The CED's essential knowledge says an interval plus its inversion equals a perfect [octave](/ap-music-theory/key-terms/octave "fv-autolink"), and the P4/P5 pair is the textbook example. Dividing an octave at the fifth automatically creates a fourth on top.

### [Consonance (Unit 1)](/ap-music-theory/key-terms/consonance)

The perfect fourth is classified as a [consonant](/ap-music-theory/key-terms/consonant "fv-autolink") interval when you're just labeling interval qualities. Keep in mind that in strict two-voice writing it gets treated more carefully than thirds and sixths, so context determines how "stable" it actually sounds.

### [Diatonic Scale (Unit 1)](/ap-music-theory/key-terms/diatonic-scale)

In a major scale, every fourth between scale degrees is perfect except one. Scale degree 4 up to scale degree 7 (like F to B in C major) is an augmented fourth, the tritone. Knowing where that one exception hides is a classic MCQ trap.

## On the AP Exam

Expect the perfect fourth in multiple-choice questions about interval identification and inversion, in both notated and aural formats (that's the two-part demand of LO 2.6.A). Practice questions hit the inversion relationship head-on, asking things like "What is the inversion of a perfect fifth?" (answer: a perfect fourth) and "How can you interpret interval inversions in two ways?" (move the lower note up an octave, or split an octave into two intervals). Aurally, sight-singing and melodic dictation both reward being able to hear a fourth instantly, since leaps of a fourth are everywhere in melodies. Your job on the exam is to identify the interval, invert it correctly, and keep the quality logic straight: perfect stays perfect, and the sizes always add to nine.

## Perfect Fourth vs Perfect Fifth

These two get mixed up because they're inversions of each other, meaning they contain the same two note names flipped. C up to G is a perfect fifth (seven half steps); G up to C is a perfect fourth (five half steps). The fifth sounds more open and stable; the fourth sounds slightly tenser, like it wants to resolve. On notation questions, count letter names carefully. C-D-E-F-G is five letters (a fifth), while C-D-E-F is four (a fourth). If you accidentally count half steps when you meant letter names, you'll land on the wrong one.

## Key Takeaways

- A perfect fourth spans four letter names and five half steps, like C up to F or G up to C.
- The perfect fourth is the inversion of the perfect fifth, and per the CED, perfect intervals remain perfect when inverted.
- An interval plus its inversion equals a perfect octave, and their sizes always sum to nine, so a fourth inverts to a fifth (4 + 5 = 9).
- In a major scale, every fourth between scale degrees is perfect except scale degree 4 up to scale degree 7, which is an augmented fourth (the tritone).
- LO 2.6.A requires you to identify inversions in both performed and notated music, so practice hearing the difference between a fourth and a fifth, not just spelling it.

## FAQs

### What is a perfect fourth in AP Music Theory?

A perfect fourth is an interval spanning four letter names and five half steps, such as C up to F. It's tested in Topic 2.6 as the inversion of the perfect fifth, since perfect intervals stay perfect when inverted and the sizes sum to nine.

### Is every fourth in a major scale a perfect fourth?

No. In a major scale, the fourth from scale degree 4 up to scale degree 7 (F to B in C major) is an augmented fourth, six half steps instead of five. Every other diatonic fourth in the major scale is perfect.

### What is the inversion of a perfect fourth?

A perfect fifth. Move the lower note of the fourth up an octave and you get a fifth, the quality stays perfect, and the sizes add to nine (4 + 5). This exact relationship shows up in inversion MCQs.

### How is a perfect fourth different from a perfect fifth?

A perfect fourth covers four letter names and five half steps (C to F), while a perfect fifth covers five letter names and seven half steps (C to G). They're inversions of each other, so flipping one gives you the other.

### Is a perfect fourth consonant or dissonant?

For interval labeling on the AP exam, the perfect fourth counts as consonant. In strict two-voice counterpoint contexts it's handled more cautiously than thirds and sixths, but for Unit 2 identification questions, treat it as a consonance with perfect quality.

## 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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