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Interval (music)
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Interval (music)

In music theory, an interval is the difference (a ratio or logarithmic measure) in pitch between two notes and often refers to those two notes themselves (otherwise known as a dyad). An interval class is measured by the shortest distance possible between its two pitch classes.

Intervals may be labelled according their pitch ratios, as is commonly used in just intonation. Intervals may also be labelled according to their diatonic functionality, as is commonly done for tonal music, and according to the number of notes they span in a diatonic scale. The interval of a note from its tonic is its scale degree, thus the fifth degree of a scale is a fifth from its tonic. For atonal music, such as that written using the twelve tone technique or serialism, integer notation is often used, such as in musical set theory. Finally, it is also possible to label intervals using the logarithmic measure of centss, as is used to compare other intervals with those of twelve tone equal temperament.

Intervals may also be described as narrow and wide or small and large, consonant and dissonant or stable and unstable, simple and compound, vertical (or harmonic) and linear (or melodic), and, if linear as steps or skips. Simple intervals are those which lie within an octave and compound are those which are larger than a single octave. Thus a tenth is known as a compound third. Finally, intervals may be labelled with or modified by the addition of perfect, major, minor, augmented, and diminished before the number of notes apart (for instance, augmented fourth). Perfect intervals are never major or minor and major and minor intervals are never perfect. Major and minor intervals are one semitone above, or below, their minor and major counterparts, respectively (see minor second below). Augmented and diminished intervals are raised or lowered a step and any interval may be augmented or diminished and may even be double augmented or diminished. Linear intervals are successive pitches while vertical intervals are simultaneous. Steps are linear intervals between consecutive scale degrees while skips are not.

It is important to note that while intervals named by their harmonic functions, for instance, a major second, may be described by a ratio, cent, or integer, not every interval described by these more general terms may be described with the harmonic function name. For instance, all major seconds (in twelve tone equal temperament) are 200 cents, but not every interval of 200 cents is a major second. See: enharmonic.

Table of contents
1 Simple diatonic intervals
2 Ordered and unordered pitch and pitch class intervals
3 Interval cycles
4 Interval strength and root
5 Other intervals

Simple diatonic intervals

Below are listed the most commonly used harmonic function, ratio, integer, cents, and relative consonance or dissonance of common diatonic simple intervals. There are many other intervals and ratios, some of which follow.

Common simple intervals

Augmented and diminished intervals

Along with all seconds and sevenths, all augmented and diminished intervals are considered dissonant. However, in twelve tone
equal temperament, most intervals, when augmented or diminished, are enharmonically equivalent to another interval. For example, a diminished minor second is a unison and thus only the fourth and fifth are commonly altered.

Abbreviated

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 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.

Thus a minor second is m2, a perfect fifth is P5, an diminished 3rd is d3, and an augmented fourth is A4.

The intervals in the chromatic scale are (in ascending melodic order): P1, m2, M2, m3, M3, P4, π, P5, m6, M6, m7, M7, P8.

Generations of intervals

The intervals can be divided into five "generations", which correspond to negative powers of two:
Zeroth generation (1+2−0): P1, P8.
First generation (1+2−1): P4, P5.
Second generation (1+2−2): M3, m3, M6, m6.
Third generation (1+2−3): M2, m7.
Fourth generation (1+2−4): m2, M7, π.

Each successive generation is more dissonant than the previous one.

Here is the derivation of each generation from the previous one: Start with the octave's ratio, 2:1. Multiply each of its two numbers by two, viz. 4:2. Then stick the missing number in the middle, viz. 4:3:2. This breaks up into a pair of ratios — 4:3 and 3:2. The minor one is 4:3 and the major one is 3:2. These are the perfect fourth and the perfect fifth, respectively, and they are the first generation.

Now take the perfect fifth's ratio, 3:2. Multiply each of its two numbers by two, viz. 6:4. Then stick the missing number in the middle, viz. 6:5:4. This breaks up into a pair of ratios — 6:5 and 5:4. The minor one is 6:5 and the major one is 5:4. These are the minor third and major third, respectively. Their inversions are 5:3 and 8:5, which are the major sixth and the minor sixth, respectively. So these are the second generation: M3, m3, M6, m6.

Now take the major third's ratio, 5:4. Multiply each of its two numbers by two, viz. 10:8. Then stick the missing number in the middle, viz. 10:9:8. This breaks up into a pair of ratios — 10:9 and 9:8. The minor one is 10:9 and the major one is 9:8. Both of these are whole-tones, i.e. major seconds. The inversion of 9:8 is 16:9, a minor seventh. So these are the third generation: M2, m7.

Now take the whole-tone's ratio, 9:8. Multiply each of its two numbers by two, viz. 18:16. Then stick the missing number in the middle, viz. 18:17:16. This breaks up into a pair of ratios — 18:17 and 17:16. The minor one is 18:17 and the major one is 17:16. Both of these are semitones, i.e. minor seconds. The inversion of 18:17 is 17:9, a major seventh. The last interval is the tritone. The tritone is ideally equal to the square root of two, which is irrational, but can be approximated by adding a semitone to a perfect fourth:

or
which is the inversion of 17:12. 24:17 has the same denominator as 18:17, and 17:12 has the same numerator as 17:16. So these are the intervals of the fourth generation: m2, M7, π.

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. Using integer notation and modulo 12, ordered pitch interval, ip, may be defined, for any two pitches x and y, as:

and: the other way.

One can also measure the distance between two pitches without taking into account direction with the unordered pitch interval, similar to the interval of tonal theory. This may be defined as:

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. Thus the ordered pitch class interval, i<x, y>, may be defined as: mod 12, of course.

For unordered pitch class interval see interval class.

Interval cycles

Interval cycles, "unfold 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. "These interval cycles play a fundamental role in the harmonic organization of post-diatonic music and can easily be identified by naming the cycle." (Perle, 1990)

Interval cycles are symmetrical and thus non-diatonic. However, a seven pitch segment of C5 will produce the diatonic major scale. Cyclic tonal progressions in the works of Romantic composers such as Mahler and Wagner form a link with the cyclic pitch successions in the atonal music of Modernists such as Bartok, Scriabin, Varese, and the Vienna school. At the same time, these progressions signal the end of tonality.

Source

Interval strength and root

Interval strength

David Cope suggests the concept of interval strength, in which an interval's strength is determined by its approximation to a lower, stronger, or higher, weaker, position in the harmonic series.

Interval roots

Hindemith and David Cope both suggest the concept of interval roots. 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.

Source

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.

For the mathematical use of the word "interval", see interval (mathematics).