An interval is the distance in pitch between two notes, and you describe it with two parts: size (the number like 2nd, 3rd, 5th) and quality (major, minor, perfect, augmented, or diminished). Size comes from counting letter names, and quality comes from the exact number of half steps.
Why This Matters for the AP Music Theory Exam
Intervals are the building block for almost everything that comes later in AP Music Theory, including triads, seventh chords, and harmonic analysis. When you can name size and quality quickly, you can spell chords, label scale steps, and check your part writing without slowing down.
This skill shows up in two ways on the exam. You read and describe intervals in notated music, and you recognize them by ear in performed music. Aural interval recognition feeds directly into melodic dictation, where you notate a melody from a given starting pitch, and into sight-singing, where you sing a melody from a score. Note that the aural section will not ask you to name a specific key letter, but it does expect you to hear how pitches relate.
Key Takeaways
- Size is counted by letter names: C up to E is a 3rd, C up to G is a 5th, no matter the accidentals.
- A major scale gives all major intervals except the 4th, 5th, and octave, which are perfect.
- Lower a major interval by a half step to get minor; perfect intervals never become minor.
- Augmented adds a half step above major or perfect; diminished subtracts a half step below minor or perfect.
- Harmonic intervals sound at the same time; melodic intervals sound one after another, with a step moving to a neighboring letter and a leap moving farther.
- Consonant intervals feel stable; dissonant intervals feel unstable and pull toward resolution.
Naming Intervals: Size and Quality
Every interval has a size and a quality.
Size is the number, like 2nd, 3rd, 4th, up to 7th. You find size by counting letter names from the bottom note to the top note, including both ends. C to E spans C, D, E, so it is a 3rd. E to G spans E, F, G, so it is also a 3rd. Accidentals do not change the size, only the quality.
Quality is the type of interval: major, minor, perfect, augmented, or diminished. Together, size and quality give a full name like major second or diminished seventh.
A fast way to find quality is to compare against a major scale built on the lower note.
- A major scale produces major intervals on the 2nd, 3rd, 6th, and 7th.
- The 4th, 5th, and octave (and unison) are perfect, not major.
So C up to E is a major 3rd, and C up to G is a perfect 5th.
Minor, Augmented, and Diminished
Once you know the major and perfect intervals, the rest follow predictable steps:
- Minor is one half step smaller than major. C to E is a major 3rd, so C to Eb is a minor 3rd. There is no such thing as a minor 4th, 5th, or octave, because those are perfect, not major.
- Augmented is one half step larger than a major or perfect interval. C to E# is an augmented 3rd, and C to G# is an augmented 5th.
- Diminished is one half step smaller than a minor or perfect interval. C to Ebb is a diminished 3rd, and C to Gb is a diminished 5th.
Augmented and diminished intervals show up less often than major, minor, and perfect ones, but they matter. You will likely meet augmented 2nds and augmented 4ths later when you study part writing, where they are usually avoided.
Shorthand Notation
Intervals are often written with a letter for quality and a number for size:
- M = major, m = minor, P = perfect, A or + = augmented, d or ° = diminished
So a perfect fifth is P5 and a minor third is m3.
Unique Names
A few intervals have special names:
- Unison (prime): two notes on the same pitch.
- Octave: two notes that share a letter name an octave apart.
- Tritone: the interval three whole steps apart, spelled as either an augmented 4th or a diminished 5th.
Enharmonic Equivalents
Two intervals are enharmonic equivalents when they sound the same but are spelled with different letter names. The clearest example is the tritone: D up to G# is an augmented 4th, and D up to Ab is a diminished 5th. They sound identical, but the spelling decides the name. This is why you always check letter names for size before deciding quality.
Harmonic and Melodic Intervals
Intervals come in two contexts:
- Harmonic intervals are the distance between two pitches sounding at the same time.
- Melodic intervals are the distance between two pitches that sound one after another.
Melodic intervals split into two general types. A step (conjunct motion) moves to a neighboring letter name, like C up to D. A leap (disjunct motion) moves farther than a step, like C up to E. Recognizing steps versus leaps helps you describe melodic shape and makes dictation faster.
Interval Inversion
Interval inversion means moving the lower note up an octave so it becomes the top note. An interval plus its inversion always adds up to a perfect octave.
Two patterns make inversion easy:
- The two sizes always add up to 9. A 2nd inverts to a 7th, a 3rd inverts to a 6th, a 4th inverts to a 5th.
- Qualities flip: major becomes minor and minor becomes major, augmented becomes diminished and diminished becomes augmented, and perfect stays perfect.
For example, an A5 inverts to a d4, and an m6 inverts to a M3.
Consonant and Dissonant Intervals
Some intervals sound stable and settled, while others sound tense and want to move. Stable intervals are consonant, and unstable ones are dissonant.
- Consonant: the octave, perfect 5th, and major and minor 3rds and 6ths.
- Dissonant: major and minor 2nds, the tritone, major and minor 7ths, and any augmented or diminished interval.
The perfect 4th is a special case. In some contexts it sounds stable, but in others it pulls toward the perfect 5th and behaves like a dissonance.
Dissonance is not a problem to avoid. Good writing often uses dissonance to build tension before resolving to consonance, like a harmonic diminished fifth resolving inward to a third. Keep in mind these consonance and dissonance categories come from the Western tradition tested on this exam, and other musical traditions define stability differently.
How to Use This on the AP Music Theory Exam
Notated Music
When you see an interval in a score, count size first by letter names, then check the accidentals to decide quality. Do this in order so you do not let a sharp or flat trick you into the wrong size. Use the major-scale comparison: build the major scale on the lower note, see what the top note would be, then adjust for minor, augmented, or diminished.
Aural Recognition and Dictation
For melodic dictation, you notate a melody starting from a given pitch, so you need to hear intervals and translate them into the right notes. Practice hearing each interval as a distance, then writing the correct letter and accidental. Singing on solfege (do re mi) is a reliable way to build this skill, and connecting intervals to songs you already know helps you recognize them under pressure.
Sight-Singing
In sight-singing you get a starting pitch and a score, so you sing each interval accurately to move from note to note. The same interval fluency that helps you take dictation helps you sing on pitch.
Common Trap
Spelling matters. An augmented 4th and a diminished 5th sound the same, but they are different intervals on paper. Always confirm size from letter names before naming quality, because the exam can give you correct sounds with tricky spellings.
Common Misconceptions
- Accidentals do not change interval size. C to E and C to Eb are both 3rds; only the quality changes.
- There is no minor 4th, minor 5th, or minor octave. Those are perfect intervals, so their altered forms are augmented or diminished, not minor.
- Minor intervals are not the same as the minor scale. C to Db is a minor 2nd even though Db is not in the C minor scale.
- The tritone is one sound with two spellings. It can be an augmented 4th or a diminished 5th depending on letter names, even though it sounds the same.
- Inversion qualities flip, but perfect stays perfect. A perfect interval inverts to another perfect interval, while major and minor swap and augmented and diminished swap.
- Dissonance is not bad. It creates tension that resolves to consonance and is a normal part of good writing.
Related AP Music Theory Guides
Vocabulary
The following words are mentioned explicitly in the College Board Course and Exam Description for this topic.Term | Definition |
|---|---|
augmented | A quality designation for intervals that are one semitone larger than perfect or major intervals (e.g., augmented fourth, augmented second). |
consonance | The quality of intervals that are inherently stable and have no natural inclination to move or resolve to other sounds. |
diminished | A quality designation for intervals that are one semitone smaller than perfect or minor intervals (e.g., diminished fifth, diminished seventh). |
dissonance | The quality of intervals that are inherently unstable and have a natural inclination to move or resolve to more stable sounds. |
enharmonic equivalents | Tones of the same pitch spelled differently according to their musical contexts, such as C# and Db. |
harmonic interval | An interval formed between two simultaneous pitches sounding at the same time. |
interval | The distance in pitch between two notes, designated by both size (such as second or fifth) and quality (major, minor, perfect, diminished, or augmented). |
leap | A melodic interval larger than a step, traversing pitches that are not adjacent letter names. |
major | A quality designation for intervals, indicating a specific interval size larger than minor intervals (e.g., major second, major third). |
melodic interval | The distance in pitch between two consecutive notes in a melody. |
minor | A quality designation for intervals, indicating a specific interval size smaller than major intervals (e.g., minor second, minor third). |
perfect | A quality designation for certain intervals (unison, fourth, fifth, octave) that are considered inherently stable and consonant. |
step | A melodic interval that traverses adjacent pitches with neighboring letter names. |
tritone | A unique interval name designating an augmented fourth or diminished fifth, spanning six semitones. |
unison | An interval between two identical pitches, also called prime, representing no distance between notes. |
Frequently Asked Questions
What is interval size and quality in music?
Interval size is the numbered distance between two note names, like a 2nd, 3rd, or 5th. Interval quality tells the exact sound and spelling: major, minor, perfect, augmented, or diminished.
How do you count interval size?
Count the letter names from the lower note to the upper note, including both notes. C to E is a 3rd because you count C, D, E; accidentals affect quality, not size.
What is the difference between major and minor intervals?
Major intervals are the 2nd, 3rd, 6th, and 7th found in a major scale. Lowering one of those major intervals by a half step makes it minor.
Which intervals are perfect intervals?
Unisons, 4ths, 5ths, and octaves are perfect intervals when they match the major-scale form. Perfect intervals can become augmented or diminished, but they do not become major or minor.
What is a tritone?
A tritone is an interval three whole steps wide. It can be spelled as an augmented 4th or a diminished 5th, depending on the note names and musical context.
What is the difference between harmonic and melodic intervals?
A harmonic interval happens when two notes sound at the same time. A melodic interval happens when the notes sound one after another, like in a melody.