A musical interval is the distance in pitch between two notes. Intervals are fundamental units of melody, harmony, scales, chords and musical tuning because relationships between pitches are more musically significant than isolated pitch names. An interval can be measured by counting the number of letter names encompassed and then identifying its quality. The basic diatonic interval numbers are unison, second, third, fourth, fifth, sixth, seventh and octave.
For example, C to E is some type of third because the letter names C, D and E encompass three positions, while C to G is some type of fifth because C, D, E, F and G encompass five positions. The precise interval also depends on the number of semitones or the underlying tuning relationship. Thus C to E is a major third, C to E flat is a minor third, and C to E sharp is an augmented third, despite the notes occupying related pitch distances in some enharmonic contexts.
Interval quality provides a more precise description than interval number alone. Perfect intervals include the unison, fourth, fifth and octave, while major and minor qualities apply to seconds, thirds, sixths and sevenths. An interval can also be augmented or diminished when it is expanded or contracted relative to its normal perfect, major or minor form. A C to F relationship is a perfect fourth, while C to F sharp is an augmented fourth.
C to B is a major seventh, while C to B flat is a minor seventh. C to G flat is a diminished fifth. Enharmonic spelling matters because C to F sharp and C to G flat contain the same number of equal tempered semitones but are theoretically different intervals and can perform different harmonic functions. Interval terminology therefore communicates both numerical distance and notational or functional structure.
Intervals can be classified as melodic or harmonic. A melodic interval occurs when two notes are sounded successively, whereas a harmonic interval occurs when they sound simultaneously. The same two pitches can form either type of interval, but their perceptual and musical behaviour may differ because simultaneous frequencies interact through consonance, dissonance and acoustic interference. Intervals can also be described as ascending or descending when considering melodic direction.
Compound intervals extend beyond an octave and can be reduced to a corresponding simple interval for some analytical purposes. For example, C to the D above the next octave is a major ninth, which is a compound form of a major second. Compound intervals are important in orchestration, vocal writing and instrumental arrangement because large registral distances can substantially affect clarity, resonance and perceived harmonic density.
Intervals have traditionally been classified according to consonance and dissonance, although these classifications vary by historical period, musical culture and theoretical system. In common Western theory, octaves and fifths are strongly consonant, while major and minor thirds and sixths are generally treated as consonant in tonal harmony. Seconds, sevenths and tritones are commonly treated as dissonant, although their treatment depends heavily on context.
The tritone, which spans three whole tones, can be represented as an augmented fourth or diminished fifth and has historically received particular theoretical attention. Consonance is not simply a property that determines whether an interval is pleasant or unpleasant. Its musical function can depend on preparation, resolution, voice leading, register, dynamics, timbre, tuning and stylistic convention. An interval considered dissonant in one context can become an ordinary structural component in another.
Intervals form the structural basis of scales and chords. A major scale can be described through its sequence of whole tones and semitones, while a major triad consists of a major third above the root and a minor third above that third, producing a perfect fifth between root and fifth. Chord quality can therefore be analysed through combinations of intervals.
Melodic contour is likewise governed by intervals because a melody can be described not only through absolute notes but through the succession of pitch distances between them. Intervals also define transposition. If a melody is moved upward by a major third, each pitch is shifted by the same interval, preserving its intervallic structure. Interval recognition is consequently central to ear training, composition, analysis, improvisation, tuning and instrumental technique.
Interval measurement is closely connected to semitones and tuning systems. In twelve tone equal temperament, an octave is divided into twelve equal semitone steps, allowing interval sizes to be represented numerically. A minor second spans one semitone, a major second two, a minor third three, a major third four, a perfect fourth five, a tritone six, a perfect fifth seven, a minor sixth eight, a major sixth nine, a minor seventh ten and a major seventh eleven.
The octave spans twelve semitones. These measurements describe equal tempered pitch relationships, but other tuning systems use different frequency ratios or unequal divisions. Just intonation, for example, can represent intervals through simple frequency ratios, while historical temperaments distribute tuning discrepancies differently. Therefore a theoretical interval name does not necessarily imply an identical acoustic frequency ratio in every musical system.
Intervals also have specialised analytical uses. In counterpoint, intervallic relationships between simultaneous voices help determine acceptable consonances, dissonances and voice leading. In harmony, intervals reveal chord construction, inversions and tensions. In melody, characteristic intervals can define motifs and contribute to musical identity. In acoustics, intervals correspond to relationships between frequencies and influence beating and harmonic interaction. In atonal and post tonal theory, interval classes may be used to group intervals according to equivalence under inversion and octave displacement.
An interval class treats intervals such as a minor third and major sixth as related because they contain the same number of semitone classes under octave equivalence. This demonstrates that interval terminology can operate at several analytical levels, from practical notation to abstract pitch class theory.
A complete understanding of intervals therefore requires attention to notation, pitch distance, quality, direction, register, tuning and musical function. The same sounding distance can receive different theoretical spellings depending on harmonic context, and the same named interval can have different perceptual effects depending on register and timbre. Intervals are neither merely mathematical measurements nor isolated theoretical labels. They are the relationships from which scales, melodies, harmonies, chord structures, counterpoint and many compositional techniques are constructed. Learning interval names provides a vocabulary for describing music precisely, while recognising intervals aurally develops practical skills needed by singers, instrumentalists, composers, arrangers, conductors and analysts.
