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In music theory, the term interval describes the relationship between the pitches of two notes.

Intervals may be described as:
  • vertical (or harmonic) if the two notes sound simultaneously
  • linear (or melodic), if the notes sound successively.


Interval class is a system of labelling intervals when the order of the notes is left unspecified, therefore describing an interval in terms of the shortest distance possible between its two pitch classes.

Frequency ratios

Intervals may be labelled according to the ratio of frequencies of the two pitches. Important intervals are those using the lowest integers, such as 1:1 (unison or prime), 2:1 (octave), 3:2 (perfect fifth), 4:3 (perfect fourth), etc. This system is frequently used to describe intervals in both Western and non-Western music. This method is also often used in just intonation, and in theoretical explanations of equal-tempered intervals used in European tonal music, to explain them through their approximation of just intervals.

Interval number and quality

Number

Interval names
In Western harmonic theory, intervals are labeled according to the number of scale steps or staff positions they encompass, as shown at right.

Intervals larger than an octave are called compound intervals; for example, a tenth is known as a compound third. The quality of the compound interval is determined by the quality of the interval on which it is based. For example, a perfect eleventh is the same as a compound perfect fourth.

Intervals larger than a thirteenth seldom need to be spoken of, most often being referred to by their compound names, for example "two octaves plus a fifth" rather than "a 19th".

The name or the label of an interval is determined by counting the number of degrees between the two notes beginning with one for the lower note. The number of degrees between F and B for example is 4, therefore the interval is a fourth.

Quality

The name of any interval is further qualified using the terms perfect, major, minor, augmented, and diminished. This is called its interval quality.

Number of
semitones
name enharmonic notes
0 Perfect Unison (P1) Diminished second (dim2)
1 Minor second (m2) Augmented unison (aug1)
2 Major second (M2) Diminished third (dim3)
3 Minor third (m3) Augmented second (aug2)
4 Major third (M3) Diminished fourth (dim4)
5 Perfect fourth (P4) Augmented third (aug3)
6 Tritone Augmented fourth (aug4)

Diminished fifth (dim5)
7 Perfect fifth (P5) Diminished sixth (dim6)
8 Minor sixth (m6) Augmented fifth (aug5)
9 Major sixth (M6) Diminished seventh (dim7)
10 Minor seventh (m7) Augmented sixth (aug6)
11 Major seventh (M7) Diminished octave (dim8)
12 Perfect octave (P8) Augmented seventh (aug7)


It is possible to have doubly-diminished and doubly-augmented intervals, but these are quite rare.

The name of an interval cannot, in general, be determined by counting semitones alone. For example, there are four semitones between B and E♭, however this interval is a diminished fourth—an interval found between the seventh and third degrees of the harmonic minor scale—rather than a major third. In equal-tempered tuning, as on a piano, these intervals are indistinguishable by sound, but the diatonic function of the notes incorporated might be very different.

  • Major/minor:
Major/minor intervals are so called because rather than containing one number of semitones they are at a lower, minor, or higher, major, number. For example, the minor sixth is 8 semitones and the major sixth is 9 semitones.

  • Perfect:
Perfect intervals are so called because they are neither minor nor major (such as thirds, which are either minor or major) but perfect. For example, the perfect fifth is 7 semitones and may not be major or minor.

  • Augmented/diminished:
Augmented/diminished intervals are so called because rather than containing one number of semitones they are at a lower, diminished, or higher, augmented, number. For example, the diminished second is 0 semitones and the augmented second is 3 semitones, while the augmented fourth and the diminished fifth are both 6 semitones.

Diatonic and chromatic intervals

A diatonic interval is an interval formed by two notes of a diatonic scale. The table on the right depicts all diatonic intervals for C major.

Shorthand notation

Intervals are often abbreviated with a P for perfect, m for minor, M for major, d for diminished, A for augmented, followed by the diatonic interval number. The indication M and P are often omitted. The octave is P8, and a unison is usually referred to simply as "a unison" but can be labeled P1. The tritone, an augmented fourth or diminished fifth is often π or TT. Examples:
  • m2: minor second
  • M3: major third
  • P5: perfect fifth
  • m9: minor ninth.


For use in describing chords, the sign + is used for augmented and ° for diminished. Furthermore the 3 for the third is often omitted, and for the seventh, the plain form stands for the minor interval, while the major is indicated by maj. So for example:
  • m: minor third (with perfect fifth)
  • 7: minor seventh (with major third and perfect fifth)
  • °7: diminished seventh (with minor third and diminished fifth)
  • maj7: major seventh (with major third and perfect fifth)
  • +5: augmented fifth (with major third)
  • °5: diminished fifth (with minor third).


Enharmonic intervals

Two intervals are considered to be enharmonic, or enharmonically equivalent, if they both contain the same pitches spelled in different ways; that is, if the notes in the two intervals are themselves enharmonically equivalent. Enharmonic intervals span the same number of semitones. For example, as shown in the matrix below, F –A (a major third), G –B (also a major third), F –B (a diminished fourth), and G –A (a double augmented second) are all enharmonically equivalent — and they all span four semitones.

step 1 2 3 4
major third F   A  
major third   G   B
diminished fourth F     B
double augmented second   G A  


Steps and skips

Linear (melodic) intervals may be described as steps or skips in a diatonic context. Steps are linear intervals between consecutive scale degrees while skips are not, although if one of the notes is chromatically altered so that the resulting interval is three semitones or more (e.g. C to D♯), that may also be considered a skip. However, the reverse is not true: a diminished third, an interval comprising two semitones, is still considered a skip.

The words conjunct and disjunct refer to melodies composed of steps and skips, respectively.

Pitch-class intervals

Post-tonal or atonal theory, originally developed for equal tempered European classical music written using the twelve tone technique or serialism, integer notation is often used, most prominently in musical set theory. In this system intervals are named according to the number of half steps, from 0 to 11, the largest interval class being 6.

Ordered and unordered pitch and pitch-class intervals

In atonal or musical set theory there are numerous types of intervals, the first being ordered pitch interval, the distance between two pitches upward or downward. For instance, the interval from C to G upward is 7, but the interval from G to C downward is −7. One can also measure the distance between two pitches without taking into account direction with the unordered pitch interval, somewhat similar to the interval of tonal theory.

The interval between pitch classes may be measured with ordered and unordered pitch-class intervals. The ordered one, also called directed interval, may be considered the measure upwards, which, since we are dealing with pitch classes, depends on whichever pitch is chosen as 0. For unordered pitch-class interval see interval class.

Generic and specific intervals

In diatonic set theory, specific and generic intervals are distinguished. Specific intervals are the interval class or number of semitones between scale degrees or collection members, and generic intervals are the number of scale steps between notes of a collection or scale.

Cents

The standard system for comparing intervals of different sizes is with cents. This is a logarithmic scale in which the octave is divided into 1200 equal parts. In equal temperament, each semitone is exactly 100 cents.The value in cents for the interval f1 to f2 is 1200×log2(f2/f1).

Comparison of different interval naming systems

# semitones

Interval

class
Generic

interval
Common

diatonic name
Comparable

just interval
Comparison of interval width in cents
equal
temperament
just
intonation
quarter-comma
meantone
0 0 1 perfect unison 1:1 0 0 0
1 1 2 minor second 16:15 100 112 117
2 2 2 major second 9:8 200 204 193
3 3 3 minor third 6:5 300 316 310
4 4 3 major third 5:4 400 386 386
5 5 4 perfect fourth 4:3 500 498 503
6 6 4

5
augmented fourth

diminished fifth
45:32

64:45
600 590

610
579
621
7 5 5 perfect fifth 3:2 700 702 697
wolf fifth 737
8 4 6 minor sixth 8:5 800 814 814
9 3 6 major sixth 5:3 900 884 889
10 2 7 minor seventh 16:9 1000 996 1007
11 1 7 major seventh 15:8 1100 1088 1083
12 0 1

8
perfect octave 2:1 1200 1200 1200


It is possible to construct just intervals which are closer to the equal-tempered equivalents, but most of the ones listed above have been used historically in equivalent contexts. In particular the tritone (augmented fourth or diminished fifth), could have other ratios; 17:12 (603 cents) is fairly common. The 7:4 interval (the harmonic seventh) has been a contentious issue throughout the history of music theory; it is 31 cents flatter than an equal-tempered minor seventh. Some assert the 7:4 is one of the blue notes used in jazz.

In the diatonic system, every interval has one or more enharmonic equivalents, such as augmented second for minor third.

Consonant and dissonant intervals

Consonance and dissonance are relative terms referring to the stability, or state of repose, of particular musical effects. Dissonant intervals would be those which cause tension and desire to be resolved to consonant intervals.

These terms are relative to the usage of different compositional styles.
  • In atonal music all intervals (or interval classes) are considered equally consonant melodically and harmonically.
  • In the Middle Ages, only the octave and perfect fifth were considered consonant harmonically.
  • In 16th-century usage, perfect fifths and octaves, and major and minor thirds and sixths were considered harmonically consonant, and all other intervals dissonant. In the common practice period, it makes more sense to speak of consonant and dissonant chords, and certain intervals previously thought to be dissonant (such as minor sevenths) became acceptable in certain contexts. However, 16th-century practice continued to be taught to beginning musicians throughout this period.
  • Hermann von Helmholtz (1821–1894) defined a harmonically consonant interval as one in which the two pitches have an overtone in common (specifically excluding the seventh harmonic). This essentially defines all seconds and sevenths as dissonant, while perfect fourths and fifths, and major and minor thirds and sixths, are consonant.
  • Pythagoras defined a hierarchy of consonance based on how small the numbers were which express the ratio. 20th-century composer and theorist Paul Hindemith's system has a hierarchy with the same results as Pythagoras's, but defined by fiat rather than by interval ratios, to better accommodate equal temperament, all of whose intervals (except the octave) would be dissonant using acoustical methods.
  • Lucy tuning (1990), uses a system of ScaleCoding, whereby intervals which are closer on the spiral of fourths and fifths are considered to be more consonant than those which are separated by a greater number of steps of fourths and fifths..
  • David Cope (1997) suggests the concept of interval strength, in which an interval's strength, consonance, or stability is determined by its approximation to a lower and stronger, or higher and weaker, position in the harmonic series. See also: Lipps-Meyer law.


All of the above analyses refer to vertical (simultaneous) intervals.

Inversion

An interval may be inverted, by raising the lower pitch an octave, or lowering the upper pitch an octave (though it is less usual to speak of inverting unisons or octaves). For example, the fourth between a lower C and a higher F may be inverted to make a fifth, with a lower F and a higher C. Here are the ways to identify interval inversions:
Interval inversions
  • For diatonically-named intervals there are two rules which apply to all simple (i.e., non-compound) intervals:
    1. The number of any interval and the number of its inversion always add up to nine (four + five = nine, in the example just given).
    2. The inversion of a major interval is a minor interval (and vice versa); the inversion of a perfect interval is also perfect; the inversion of an augmented interval is a diminished interval (and vice versa); and the inversion of a double augmented interval is a double diminished interval (and vice versa).
A full example: E♭ below and C above make a major sixth. By the two rules just given, C natural below and E flat above must make a minor third.


  • For intervals identified by ratio, the inversion is determined by reversing the ratio and multiplying by 2. For example, the inversion of a 5:4 ratio is an 8:5 ratio.


  • Intervals identified by integer can be simply subtracted from 12. However, since an interval class is the lower of the interval integer or its inversion, interval classes cannot be inverted.


Interval roots

Although intervals are usually designated in relation to their lower note, David Cope and Hindemith both suggest the concept of interval root. To determine an interval's root, one locates its nearest approximation in the harmonic series. The root of a perfect fourth, then, is its top note because it is an octave of the fundamental in the hypothetical harmonic series. The bottom note of every odd diatonically numbered intervals are the roots, as are the tops of all even numbered intervals. The root of a collection of intervals or a chord is thus determined by the interval root of its strongest interval.

As to its usefulness, Cope provides the example of the final tonic chord of some popular music being traditionally analyzable as a "submediant six-five chord" (added sixth chords by popular terminology), or a first inversion seventh chord (possibly the dominant of the mediant V/iii). According the interval root of the strongest interval of the chord (in first inversion, CEGA), the perfect fifth (C–G), is the bottom C, the tonic.

Interval cycles

Interval cycles, "unfold [i.e. repeat] a single recurrent interval in a series that closes with a return to the initial pitch class", and are notated by George Perle using the letter "C", for cycle, with an interval-class integer to distinguish the interval. Thus the diminished-seventh chord would be C3 and the augmented triad would be C4. A superscript may be added to distinguish between transpositions, using 0–11 to indicate the lowest pitch class in the cycle.

Other intervals

There are also a number of intervals not found in the chromatic scale or labeled with a diatonic function which have names of their own. Many of these intervals describe small discrepancies between notes tuned according to the tuning systems used. Most of the following intervals may be described as microtones.
  • A Pythagorean comma is the difference between twelve justly tuned perfect fifths and seven octaves. It is expressed by the frequency ratio 531441:524288, and is equal to 23.46 cents.
  • A syntonic comma is the difference between four justly tuned perfect fifths and two octaves plus a major third. It is expressed by the ratio 81:80, and is equal to 21.51 cents.
  • A Septimal comma is 64:63, and is the difference between the Pythagorean or 3-limit "7th" and the "harmonic 7th".
  • Diesis is generally used to mean the difference between three justly tuned major thirds and one octave. It is expressed by the ratio 128:125, and is equal to 41.06 cents. However, it has been used to mean other small intervals: see diesis for details.
  • A diaschisma is the difference between three octaves and four justly tuned perfect fifths plus two justly tuned major thirds. It is expressed by the ratio 2048:2025, and is equal to about 19.5 cents.
  • A schisma (also skhisma) is the difference between five octaves and eight justly tuned fifths plus one justly tuned major third. It is expressed by the ratio 32805:32768, and is equal to 1.95 cents. It is also the difference between the Pythagorean and syntonic commas.
    • A schismic major third is a schisma different from a just major third, eight fifths down and five octaves up, F♭ in C.
  • A quarter tone is half the width of a semitone, which is half the width of a whole tone. It is equal to 50 cents.
  • A kleisma is six major thirds up, five fifths down and one octave up, or, sometimes, the septimal kleisma 225:224.
  • A limma is the ratio 256:243, which is the semitone in Pythagorean tuning.
  • A ditone is the pythagorean ratio 81:64, two 9:8 tones.


  • Additionally, some cultures around the world have their own names for intervals found in their music. See: sargam, Balimarker


See List of Musical Intervals for more.

See Musical interval mnemonics at Wikibooks for popular musical fragments that feature common intervals

Generalizations and non-pitch uses

The term "interval" can also be generalized to other elements of music besides pitch. David Lewin's Generalized Musical Intervals and Transformations uses interval as a generic measure of distance in order to show musical transformations which can change, for instance, one rhythm into another, or one formal structure into another.

See also



Notes

  1. Aikin, Jim (2004). A Player's Guide to Chords and Harmony: Music Theory for Real-World Musicians, p.24. ISBN 0879307986.
  2. Cope, David (1997). Techniques of the Contemporary Composer, p.40–41. New York, New York: Schirmer Books. ISBN 0-02-864737-8.
  3. Kostka, Stephen; Payne, Dorothy (2008). Tonal Harmony, p.21. First Edition, 1984.
  4. Hindemith, Paul (1934). The Craft of Musical Composition. New York: Associated Music Publishers. Cited in Cope (1997), p.40-41.
  5. Perle, George (1990). The Listening Composer, p.21. California: University of California Press. ISBN 0-520-06991-9.
  6. Lewin, David (1987). Generalized Musical Intervals and Transformations, for example sections 3.3.1 and 5.4.2. New Haven: Yale University Press. Reprinted Oxford University Press, 2007. ISBN 978-0-19-531713-8
  7. Ockelford, Adam (2005). Repetition in Music: Theoretical and Metatheoretical Perspectives, p.7. ISBN 0754635732.


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