Music Intervals: A Complete Reference from Unison to Octave

Music intervals are the measured distances between two pitches, and they form the structural foundation of melody, harmony, and key dimensions and scopes of music theory. This reference covers every interval classification from unison through the octave, explains how each type is constructed and identified, and maps the practical contexts in which each interval appears. Understanding interval taxonomy is prerequisite knowledge for chord construction, voice leading, modal analysis, and ear training.

Definition and scope

An interval is defined as the pitch distance between two notes, measured in half steps (semitones) and described by a combination of a number (indicating the span across letter names) and a quality (indicating the precise size within that span). The Royal Conservatory of Music's graded curriculum, widely adopted across North American music education programs, organizes intervals by two intersecting axes: numeric size (unison through octave and beyond) and qualitative type (perfect, major, minor, augmented, diminished).

The numeric component counts the letter names spanned, inclusive of both endpoints. C to G, for example, spans five letter names (C, D, E, F, G) and is therefore a fifth regardless of any sharps or flats applied to either note. The qualitative component then specifies the exact semitone count. There are 13 distinct semitone distances from 0 (unison) through 12 (octave), and interval nomenclature assigns each a quality according to the classification system below.

Intervals are also categorized by direction (ascending or descending) and simultaneity: a melodic interval occurs when the two pitches sound consecutively, while a harmonic interval occurs when they sound simultaneously. Both types share identical numeric and qualitative labels.

How it works

The interval system operates through a two-step identification process:

1. Count letter names from the lower pitch to the upper pitch, inclusive of both. This yields the number (second, third, fourth, fifth, sixth, seventh, octave).
2. Count semitones between the two pitches. Cross-reference the semitone count against the number to determine quality.

The quality vocabulary divides across two groups:

• Perfect intervals apply to unisons (0 semitones), fourths (5 semitones), fifths (7 semitones), and octaves (12 semitones). These intervals do not carry major or minor variants; they are either perfect, augmented (one semitone wider), or diminished (one semitone narrower).
• Major/minor intervals apply to seconds, thirds, sixths, and sevenths. A major second spans 2 semitones; a minor second spans 1. A major third spans 4 semitones; a minor third spans 3. A major sixth spans 9 semitones; a minor sixth spans 8. A major seventh spans 11 semitones; a minor seventh spans 10.

The tritone occupies a special position: at 6 semitones, it is exactly half the octave. It can be notated as either an augmented fourth (e.g., C to F♯) or a diminished fifth (e.g., C to G♭), depending on the letter names involved. The Music Theory Spectrum journal (published by the Society for Music Theory) addresses tritone enharmonic equivalence extensively in harmonic analysis literature, noting that notation choice signals the expected resolution direction in tonal contexts.

Compound intervals extend beyond the octave. A ninth equals an octave plus a second; a thirteenth equals an octave plus a sixth. Jazz and extended harmony routinely use 9ths, 11ths, and 13ths as chord tones.

Common scenarios

Intervals appear in three primary functional contexts in tonal music:

Consonance and dissonance classification. Historically codified in treatises from Gioseffo Zarlino's Le istitutioni harmoniche (1558) onward, intervals are ranked by perceived stability. Perfect unisons, octaves, fifths, and fourths are classified as perfect consonances. Major and minor thirds and sixths are classified as imperfect consonances. Seconds, sevenths, and the tritone are classified as dissonances requiring resolution in counterpoint.

Chord construction. A major triad consists of a major third (4 semitones) stacked below a minor third (3 semitones), producing an outer interval of a perfect fifth (7 semitones). A minor triad reverses this stack: minor third below major third. Dominant seventh chords add a minor seventh (10 semitones) above the root, creating the tritone between the third and seventh of the chord that drives resolution.

Ear training and transcription. Music educators associate intervals with reference songs to accelerate recognition. The ascending perfect fourth appears at the opening of "Here Comes the Bride" (Wagner's Bridal Chorus); the ascending minor sixth opens Tchaikovsky's Romeo and Juliet love theme. The music theory frequently asked questions resource addresses common ear training questions about building this recognition systematically.

Decision boundaries

Interval identification requires resolving four classification boundaries that produce errors in analysis and sight-reading:

Perfect vs. augmented/diminished. A perfect fourth (C to F, 5 semitones) becomes an augmented fourth (C to F♯, 6 semitones) with one alteration. Counting letter names first prevents the common misclassification of F♯ as a "sharp fifth."

Enharmonic equivalence. C to D♭ (minor second, 1 semitone) and C to C♯ (augmented unison, 1 semitone) are enharmonically identical in equal temperament but analytically distinct. The letter-name count determines correct labeling. Music theory learning resources often address this distinction as a diagnostic checkpoint.

Ascending vs. descending direction. The interval C up to A (major sixth, 9 semitones) differs from C down to A (minor third, 3 semitones). Both relationships exist between the same two pitch classes, but direction changes the number and quality entirely.

Simple vs. compound. An interval is simple when it spans an octave or less and compound when it exceeds an octave. A major ninth and a major second share identical quality, but chord voicing and voice-leading rules treat them differently. The music theory home reference situates compound intervals within broader harmonic vocabulary frameworks used in analysis at the college and conservatory level.