---
title: "Simple Interval — AP Music Theory Definition & Guide"
description: "A simple interval spans an octave or less. Learn how to name interval size and quality, reduce compound intervals, and handle inversions on AP Music Theory."
canonical: "https://fiveable.me/ap-music-theory/key-terms/simple-interval"
type: "key-term"
subject: "AP Music Theory"
unit: "Unit 2"
---

# Simple Interval — AP Music Theory Definition & Guide

## Definition

In AP Music Theory, a simple interval is any interval that spans an octave or less (unison through octave), named by both size (second, fifth, etc.) and quality (major, minor, perfect, diminished, or augmented), as covered in Topic 2.5 (Interval Size and Quality).

## What It Is

A simple interval is any [interval](/ap-music-theory/unit-2/interval-size-quality/study-guide/HxrxB0vETN0eDp83zij1 "fv-autolink") whose size is an [octave](/ap-music-theory/key-terms/octave "fv-autolink") or smaller. That means everything from a unison (also called a prime) up through an octave counts as simple, while anything bigger (a ninth, a tenth, an eleventh) is a compound interval.

Every simple interval gets a two-part name. The size tells you the distance between letter names (a second, a [fifth](/ap-music-theory/unit-3/triad-chord-qualities-m-m-d-a/study-guide/C2Wj35yXuDEj0tYdyQcc "fv-autolink"), a seventh), and the quality tells you the exact flavor (major, minor, perfect, diminished, or augmented). So "major second" and "diminished seventh" are complete interval names, while "fifth" alone is only half the answer. A couple of intervals have special names too, like the unison and the tritone. And watch out for enharmonic equivalents, which sound identical but are spelled with different pitches. D up to G# is an augmented fourth, while D up to Ab is a diminished fifth, even though your ear can't tell them apart.

## Why It Matters

Simple [intervals](/ap-music-theory/unit-3/intro-figured-bass/study-guide/4QoPYIsEpxJDxJN6EHl5 "fv-autolink") live in [Unit 2](/ap-music-theory/unit-2 "fv-autolink") (Music Fundamentals II) under Topic 2.5, and they directly support learning objective 2.5.A, which asks you to describe the size and quality of an interval in both performed and notated music. That "performed" part matters because intervals show up on the aural sections of the exam, not just on paper.

More importantly, simple intervals are the unit of measurement for almost everything that comes later. You can't spell [triads](/ap-music-theory/unit-4/satb-voice-leading/study-guide/c71tSuvM22gVBJGNiw1V "fv-autolink"), analyze part-writing, or hear chord quality without being fast and accurate at simple intervals. They're also the key to taming compound intervals, since the standard move is to reduce a compound interval down to its simple version, work with it, then expand back if needed.

## Connections

### Compound Intervals (Unit 2)

A compound interval is just a simple interval plus an octave. A major tenth is a [major third](/ap-music-theory/key-terms/major-third "fv-autolink") stretched an octave wider. Subtract 7 from the compound size and you get the simple version, which is why C2 up to E3 (a major tenth) reduces to a major third.

### [Enharmonic Equivalent (Unit 2)](/ap-music-theory/key-terms/enharmonic-equivalent)

Two simple intervals can sound identical but have different names because spelling matters. D to G# is an augmented fourth and D to Ab is a [diminished fifth](/ap-music-theory/key-terms/diminished-fifth "fv-autolink"), and the exam expects you to name the one that matches the written pitches, not just what you hear.

### Harmonic Intervals and Melodic Intervals (Unit 2)

Simple intervals come in two presentations. Harmonic intervals stack two notes at the same time, melodic intervals play them one after another. The size-and-quality name is the same either way, but you'll need to identify both versions by ear.

### Part-writing (Units 5-7)

Voice-leading rules are written in the language of simple intervals. Avoiding parallel fifths, resolving the tritone, keeping voices within an octave of each other... all of it assumes you can spot simple intervals instantly between any two voices.

## On the AP Exam

Simple intervals get tested two ways. In notated-music questions, you identify the size and quality of an interval on the staff, including tricky enharmonic spellings like augmented fourth versus diminished fifth. In aural questions, you hear two pitches (harmonic or melodic) and name the interval.

The most common multiple-choice trap involves compound intervals. A typical question gives you something like C2 to E3 and asks for its name (major tenth), or asks you to reduce a compound interval to its simple form and then invert it. Know the mechanics cold. To reduce, subtract 7 from the size. To invert, size pairs sum to 9 (a third inverts to a sixth), major flips to minor, augmented flips to diminished, and perfect stays perfect. So a major tenth reduces to a major third, which inverts to a minor sixth.

## simple interval vs Compound interval

A simple interval spans an octave or less; a compound interval spans more than an octave. The octave itself is the dividing line and counts as simple. Every compound interval is really a simple interval wearing an octave on top, so a ninth behaves like a second and a tenth behaves like a third. When the exam asks you to reduce a compound interval, you're just stripping off that extra octave by subtracting 7 from the number (10 - 7 = 3, so a tenth becomes a third). The quality never changes when you reduce.

## Key Takeaways

- A simple interval spans an octave or less, from the unison (prime) up through the octave itself.
- Every interval name has two parts, a size (second, fifth, seventh) and a quality (major, minor, perfect, diminished, or augmented).
- Reduce a compound interval to its simple form by subtracting 7 from the size, and the quality stays the same (a major tenth becomes a major third).
- When you invert a simple interval, the sizes add up to 9 and qualities flip (major to minor, augmented to diminished), except perfect intervals stay perfect.
- Enharmonic equivalents like the augmented fourth (D to G#) and diminished fifth (D to Ab) sound identical, so in notated music you must name the interval based on its spelling.
- Learning objective 2.5.A asks you to identify interval size and quality in both performed (aural) and notated music.

## FAQs

### What is a simple interval in music theory?

A simple interval is any interval that spans an octave or less, such as a major second, perfect fifth, or minor seventh. It's named by size (the number) and quality (major, minor, perfect, diminished, or augmented).

### Is an octave a simple or compound interval?

Simple. The definition is "an octave or less," so the perfect octave is the largest possible simple interval. A ninth is where compound intervals begin.

### What's the difference between a simple interval and a compound interval?

A simple interval fits within an octave; a compound interval is larger than an octave. Every compound interval reduces to a simple one by subtracting 7 from its size, so a major tenth is just a major third plus an octave.

### Do simple intervals with the same sound always have the same name?

No. Enharmonic equivalents sound identical but are spelled differently, so they get different names. D up to G# is an augmented fourth, but D up to Ab is a diminished fifth, even though both are six half steps.

### How do you invert a simple interval on the AP Music Theory exam?

The two sizes add up to 9 (a third becomes a sixth, a second becomes a seventh) and the quality flips, so major becomes minor, augmented becomes diminished, and perfect stays perfect. For example, a major third inverts to a minor sixth.

## Related Study Guides

- [2.5 Interval Size and Quality](/ap-music-theory/unit-2/interval-size-quality/study-guide/HxrxB0vETN0eDp83zij1)

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