In music theory, an interval is a combination of two notes, or the distance between their pitches. Twonote combinations are also called dyads. Although chords are often defined as sets of three or more notes, intervals are sometimes considered to be the simplest kind of chord.^{[1]} Intervals may be described as:^{[2]}

Vertical or harmonic, if the two notes sound simultaneously

Horizontal, linear, or melodic if they sound successively.
Intervals may be roughly classified as:
Interval width
The size or width of an interval can be represented using two alternative and equivalently valid methods, each appropriate to a different context: frequency ratios or cents. Frequency ratios
Main article: Interval ratio
The size of an interval between two pitches may be measured by the ratio of their frequencies. Important intervals are those measured by fractions of small numbers, such as 1:1 (unison or prime), 2:1 (octave), 3:2 (perfect fifth), 4:3 (perfect fourth), etc. (see: Limit in music). This system is frequently used to describe intervals in both Western and nonWestern music. This method is also often used in just intonation, and in theoretical explanations of equaltempered intervals used in European tonal music, to explain them through their approximation of just intervals. Cents
Main article: Cent (music)
The standard system for comparing interval 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 size in cents of the interval from frequency f_{1} to frequency f_{2} is Main intervals
The table shows the most widely used conventional names for the intervals between the notes of a chromatic scale. A semitone is any interval between two adjacent notes in a chromatic scale, a whole tone is any interval spanning two semitones, and a tritone is any interval spanning three tones, or six semitones. In Latin nomenclature, the term ditonus was originally used to indicate any interval composed of two tones, but in current English the corresponding term ditone is used with a much more specific meaning, to refer only to a major third with frequency ratio 81:64. Intervals with different names but spanning the same number of semitones may have the same width, provided that the instrument is tuned so that the 12 notes of the chromatic scale are equally spaced (a commonly used tuning system called equal temperament). However, they are defined by different notes. For instance, the interval from D to F♯ is a major third, while that from D to G♭ is a diminished fourth, although they have identical width in equal temperament (4 semitones, or 400 cents). Except for the Latin ones, the names listed here cannot be determined by counting semitones alone. The rules to determine them are explained below. Other names, determined with different naming conventions, are listed in a separate section.
Interval number and quality
In Western music theory, to name an interval we typically need to determine its number (also called diatonic number) and quality. For instance, major third (or M3) is an interval name, in which the term major (M) describes the quality of the interval, and third (3) indicates its number. Number
As shown above, intervals are labeled according to the number of staff positions they encompass, counting both the lines and gaps between lines, starting with one at the lower note. The number of staff positions from C to G for example is 5, therefore the interval CG is a fifth (denoted P5 in the figure above). In a diatonic scale, the number of staff positions coincides with the number of scale steps. This is not true for other kinds of scales, such as the chromatic scale. However, if you take away any accidentals from the notes which form an interval, by definition the notes do not change their staff position. As a consequence, the interval number for any chromatic interval is the same as the interval number of the diatonic interval formed by the two notes without accidentals. This is the reason why interval numbers are also called diatonic interval numbers, and this convention is called diatonic interval numbering. The rule to determine the diatonic number of a compound interval (an interval larger than one octave), based on the diatonic numbers of the simple intervals from which it is built is explained in a separate section. Quality
The name of any interval is further qualified using the terms perfect (P), major (M), minor (m), augmented (A), and diminished (d). This is called its interval quality. It is possible to have doublydiminished and doublyaugmented intervals, but these are quite rare, as they occur only in chromatic contexts. The quality of a compound interval is the quality of the simple interval on which it is based (see here for details). Major/minor
Major and minor intervals are socalled because certain diatonic intervals (seconds, thirds, sixths, sevenths, and their compounds) may occur in two sizes in the diatonic scale. The larger of the two versions is called major, the smaller one minor. For example, the third occurs both as three semitones away from Re, Mi, La, and Ti in the major scale (or in the C major diatonic scale, three semitones above D, E, A, and B), and four semitones away from Do, Fa, and Sol, (or C, F, and G). The smaller, threesemitone version is called the "minor third" and the larger, foursemitone one is called the "major third". Major intervals invert to minor ones, and viceversa. For example, a major second inverts to a minor seventh, and the reverse. Perfect
Perfect intervals are socalled because of their high levels of consonance, and because the inversion of a perfect interval is also perfect. Other kinds of intervals have an opposite quality with respect to their inversion. The inversion of a major interval is a minor interval, the inversion of an augmented interval is a diminished interval. Within the diatonic scale all fourths and fifths are perfect, with five and seven semitones respectively, except for one occurrence each of six semitones: the fourth between Fa and Ti (an augmented fourth), and its inversion, i.e., the fifth between Ti and Fa (a diminished fifth). Augmented/diminished
Augmented and diminished intervals are so called because they exceed or fall short of either a perfect interval, or a major/minor pair by one semitone, while having the same interval number (i.e., encompassing the same number of staff positions). Except for the augmented fourth (FaTi) and diminished fifth (TiFa), they do not appear in the diatonic scale. For instance, there is no threesemitone interval in the diatonic scale that functions as a second, and the augmented second (e.g., E♭–F♯) is three semitones wide. Example
Neither the number, nor the quality of an interval can be determined by counting semitones alone. As explained above, the number of staff positions must be taken into account as well. For example, as shown in the table below, there are four semitones between A and C♯, between A and D♭, between A♯ and D, and between A♭ and B♯, but

the interval AC♯ is called a major third (as it spans 3 staff positions),

the intervals AD♭ and A♯D are called diminished fourths (as they span 4 staff positions),

the interval A♭B♯ is called a doubly augmented second (as it spans 2 staff positions).
The diminished fourth is an interval found between the seventh and third degrees of the harmonic minor scale, while the doubly augmented second only occurs in entirely chromatic contexts. In equaltempered tuning, as on a piano, these intervals are indistinguishable by sound when played in isolation, but in musical context the diatonic function of the notes incorporated is very different. 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 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 TT. The interval qualities may be also abbreviated with perf, min, maj, dim, aug. Examples:
 m2 (or min2): minor second,
 M3 (or maj3): major third,
 A4 (or aug4): augmented fourth,
 d5 (or dim5): diminished fifth,
 P5 (or perf5): perfect fifth.
Intervals in chords
Chords are sets of three or more notes. They are typically defined as the combination of intervals starting from a common note called the root of the chord. For instance a major triad is a chord containing three notes defined by the root and two intervals (major third and major fifth). Sometimes even a single interval (dyad) is considered to be a chord.^{[1]} Chords are classified based on the quality and number of the intevals which define them. Chord qualities and interval qualities
The main chord qualities are: major, minor, augmented, diminished, halfdiminished, and dominant. The symbols used for chord quality are similar to those used for interval quality (see above). In addition, + or aug is used for augmented, ° or dim for diminished, ^{ø} for half diminished, and dom for dominant (the symbol − alone is not used for diminished). Deducing component intervals from chord names and symbols
The main rules to decode chord names or symbols are summarized below. Further details are given in the elsewhere.

For 3note chords (triads), major or minor always refer to the third interval, while augmented and diminished always refer to the fifth. The same is true for the corresponding symbols (e.g., CM means CM3, and C+ means C+5). Thus, the terms third and fifth and the corresponding symbols 3 and 5 are typically omitted.
This rule holds for 4note chords (tetrads) as well,^{[3]} provided the above mentioned qualities appear immediately after the root note. For instance, in the chord symbols CM and CM7, M refers to the interval M3, and 3 is omitted. When these qualities do not appear immediately after the root note, they should be considered interval qualities, rather than chord qualities. For instance, in Cm/M7 (minormajor seventh chord), m is the chord quality and M refers to the M7 interval. In some cases, the chord quality may refer not only to the basic triad (i.e., the third or fifth interval), but also to the following interval number. For instance, in CM7 M refers to both M3 and M7 (see specific rules below).

Without contrary information, a major third interval and a perfect fifth interval (major triad) are implied. For instance, a C chord is a C major triad (both the major third and the perfect fifth are implied). In Cm (C minor triad), a minor third is deduced according to rule 1, and a perfect fifth is implied according to this rule. This rule has one exception (see below).

When the fifth interval is diminished, the third must be minor, as a major third would produce a nontertian chord.^{[4]} This rule overrides rule 2. For instance, in Cdim7 a diminished fifth is deduced according to rule 1, and a minor third is implied according to this rule.

A plain 7 or seventh is equivalent to dom7 or dominant seventh, and stands for an extra minor seventh interval, added to the implied major triad.
The table shows the intervals contained in some of the main chords (component intervals), and some of the symbols used to denote them. The interval qualities or numbers in boldface font can be deduced from chord name or symbol by applying rule 1. In symbol examples, C is used as chord root.
Classification
The intervals contained in the table are diatonic to C major. All other intervals are chromatic to C major.
Intervals can be described, classified, or compared with each other according to various criteria. Melodic and harmonic
Main articles: Harmony and Melody
An interval can be described as

Vertical or harmonic if the two notes sound simultaneously

Horizontal, linear, or melodic if they sound successively.^{[5]}
Diatonic and chromatic
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. A chromatic interval is a nondiatonic interval formed by two notes of a chromatic scale. Aside from tritones, all intervals that are either augmented or diminished are chromatic, and the rest are diatonic. Consonant and dissonant
Consonance and dissonance are relative terms that refer to the stability, or state of repose, of particular musical effects. Dissonant intervals are those that cause tension, and desire to be resolved to consonant intervals. These terms are relative to the usage of different compositional styles.

In the Middle Ages, only the unison, octave, perfect fourth, and perfect fifth were considered consonant harmonically.

In 15th and 16thcentury usage, perfect fifths and octaves, and major and minor thirds and sixths were considered harmonically consonant, and all other intervals dissonant, including the perfect fourth, which by 1473 was described (by Johannes Tinctoris) as dissonant, except between the upper parts of a vertical sonority—for example, with a supporting third below ("63 chords").^{[6]} 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, 16thcentury 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 are that express the ratio. 20thcentury 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.

David Cope (1997) suggests the concept of interval strength^{[7]}, 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: LippsMeyer law.
All of the above analyses refer to vertical (simultaneous) intervals. Simple and compound
A simple interval is an interval spanning at most one octave. Intervals spanning more than one octave are called compound intervals.^{[8]} In general, a compound interval may be defined by a sequence or "stack" of two or more simple intervals of any kind. For instance, a major tenth (two staff positions above one octave), also called compound major third, spans one octave plus one major third. A major seventeenth (two staff positions above two octaves) is another example of compound major third, and can be built either by adding up two octaves and one major third, or four perfect fifths. Any compound interval can be always decomposed into one or more octaves plus one simple interval. For instance, a seventeenth can be always decomposed into two octaves and one major third, and this is the reason why it is called a compound major third, even when it is built using four fifths. The diatonic number DN_{c} of a compound interval formed from n simple intervals with diatonic numbers DN_{1}, DN_{2}, ..., DN_{n}, is determined by: which can also be written as: The quality of a compound interval is determined by the quality of the simple interval on which it is based. For instance, a compound major third is a major tenth (1+(81)+(31) = 10), or a major seventeenth (1+(81)+(81)+(31) = 17), and a compound perfect fifth is a perfect twelfth (1+(81)+(51) = 12) or a perfect nineteenth (1+(81)+(81)+(51) = 19). Notice that two octaves are a fifteenth, not a sixteenth (1+(81)+(81) = 15). Similarly, three octaves are a twentysecond (1+3*(81) = 22), and so on. Intervals larger than a seventeenth seldom need to be spoken of, most often being referred to by their compound names, for example "two octaves plus a fifth"^{[9]} rather than "a 19th". 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. 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 doubly augmented second) are all enharmonically equivalent. In fact, although they have a different name and staff position, F♯ and G♭ indicate the same pitch, and the same is true for A♯ and B♭. As a consequence, all these intervals span four semitones.
Size of intervals used in different tuning systems
Number of
semitones 
Name 
5limit tuning
(pitch ratio)

Comparison of interval width (in cents) 
5limit tuning 
Pythagorean
tuning 
1/4comma
meantone 
Equal
temperament 
0 
Perfect unison 
1:1 
0 
0 
0 
0 
1 
Minor second 
16:15 
112 
90 
117 
100 
2 
Major second 
9:8 
204 
204 
193 
200 
3 
Minor third 
6:5
75:64
32:27

316
(wolf) 275
294

294
318 
310
(wolf) 269

300 
4 
Major third 
5:4
512:405
32:25
81:64

386
406
(wolf) 427
408

408
384 
386
(wolf) 427

400 
5 
Perfect fourth 
4:3
675:512
27:20

498
478
520

498
(wolf) 522

503
(wolf) 462

500 
6 
Augmented fourth
Diminished fifth 
45:32
64:45

590
610

612
588 
579
621 
600 
7 
Perfect fifth 
3:2
40:27
1024:675

702
680
722

702
(wolf) 678

697
(wolf) 738

700 
8 
Minor sixth 
8:5 
814 
792 
814 
800 
9 
Major sixth 
5:3 
884 
906 
890 
900 
10 
Minor seventh 
9:5
16:9

1018
996

996 
1007 
1000 
11 
Major seventh 
15:8 
1088 
1110 
1083 
1100 
12 
Perfect octave 
2:1 
1200 
1200 
1200 
1200 
In this table, the interval widths used in four different tuning systems are compared. To facilitate comparison, just intervals as provided by 5limit tuning (see symmetric scale n.1) are shown in bold font, and the values in cents are rounded to integers. Notice that in each of the nonequal tuning systems, by definition the width of each type of interval (including the semitone) changes depending on the note from which the interval starts. This is the price paid for seeking just intonation. However, for the sake of simplicity, for some types of interval the table shows only one value (the most often observed one). In 1/4comma meantone, by definition 11 perfect fifths have a size of approximately 697 cents (700−ε cents, where ε ≈ 3.42 cents); since the average size of the 12 fifths must equal exactly 700 cents (as in equal temperament), the other one must have a size of about 700+11ε cents, which is about 738 cents (the wolf fifth); 8 major thirds have size about 386 cents (400−4ε), 4 have size about 427 cents (400+8ε), and their average size is 400 cents. In short, similar differences in width are observed for all interval types, except for unisons and octaves, and they are all multiples of ε (the difference between the 1/4comma meantone fifth and the average fifth). A more detailed analysis is provided here. Note that 1/4comma meantone was designed to produce just major thirds, but only 8 of them are just (5:4, about 386 cents). The Pythagorean tuning is characterized by smaller differences because they are multiples of a smaller ε (ε ≈ 1.96 cents, the difference between the Pythagorean fifth and the average fifth). Notice that here the fifth is wider than 700 cents, while in most meantone temperaments, including 1/4comma meantone, it is tempered to a size smaller than 700. A more detailed analysis is provided here. The 5limit tuning system uses just tones and semitones as building blocks, rather than a stack of perfect fifths, and this leads to even more varied intervals throughout the scale (each kind of interval has three or four different sizes). A more detailed analysis is provided here. Note that 5limit tuning was designed to maximize the number of just intervals, but even in this system some intervals are not just (e.g., 3 fifths, 5 major thirds and 6 minor thirds are not just; also, 3 major and 3 minor thirds are wolf intervals). The above mentioned symmetric scale 1, defined in the 5limit tuning system, is not the only method to obtain just intonation. It is possible to construct juster intervals or just intervals closer to the equaltempered equivalents, but most of the ones listed above have been used historically in equivalent contexts. In particular, the asymmetric version of the 5limit tuning scale provides a juster value for the minor seventh (9:5, rather than 16:9). Moreover, the tritone (augmented fourth or diminished fifth), could have other just ratios; for instance, 7:5 (about 583 cents) or 17:12 (about 603 cents) are possible alternatives for the augmented fourth (the latter is fairly common, as it is closer to the equaltempered value of 600 cents). The 7:4 interval (about 969 cents), also known as the harmonic seventh, has been a contentious issue throughout the history of music theory; it is 31 cents flatter than an equaltempered minor seventh. Some^{[who?]} assert the 7:4 is one of the blue notes used in jazz. For further details about reference ratios, see here. In the diatonic system, every interval has one or more enharmonic equivalents, such as augmented second for minor third. Minute intervals
Main articles: Comma and Microtone
There are also a number of minute intervals not found in the chromatic scale or labeled with a diatonic function, which have names of their own. They may be described as microtones. Except for the quarter tone, the equivalents in cents are approximate, and they can be also classified as commas, as they describe small discrepancies, observed in some tuning systems, between enharmonically equivalent notes.

A Pythagorean comma is the difference between twelve justly tuned perfect fifths and seven octaves. It is expressed by the frequency ratio 531441:524288 (23.5 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 (21.5 cents).

A septimal comma is 64:63 (27.3 cents), and is the difference between the Pythagorean or 3limit "7th" and the "harmonic 7th".

A 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 (41.1 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 (19.6 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 (2.0 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 kleisma is the difference between six minor thirds and one tritave or perfect twelfth (an octave plus a perfect fifth), with a frequency ratio of 15625:15552 (8.1 cents) ( Play (help·info)).

A septimal kleisma is six major thirds up, five fifths down and one octave up, with ratio 225:224 (7.7 cents).

A quarter tone is half the width of a semitone, which is half the width of a whole tone. It is equal to exactly 50 cents.
See Musical interval mnemonics at Wikibooks for popular musical fragments that feature common intervals Inversion
Interval inversions
Major 13th (compound Major 6th) inverts to a minor 3rd by moving the bottom note up two octaves, the top note down two octaves, or both notes one octave
A simple interval (i.e., an interval shorter than or equal to an octave) may be inverted by raising the lower pitch an octave, or lowering the upper pitch an octave. For example, the fourth from a lower C to a higher F may be inverted to make a fifth, from a lower F to a higher C. There are two rules to determine the number and quality of the inversion of any simple interval:^{[10]}
 The interval number and the number of its inversion always add up to nine (4 + 5 = 9, in the example just given).
 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; the inversion of a doubly augmented interval is a doubly diminished interval, and vice versa.
For example, the interval from C to the E♭ above it is a minor third. By the two rules just given, the interval from E♭ to the C above it must be a major sixth. Since compound intervals are larger than an octave, "the inversion of any compound interval is always the same as the inversion of the simple interval from which it is compounded."^{[11]} For intervals identified by their 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. For intervals identified by an integer number of semitones, the inversion is obtained by subtracting that number from 12. Since an interval class is the lower number selected among the interval integer and its inversion, interval classes cannot be inverted. Interval root
Although intervals are usually designated in relation to their lower note, David Cope^{[7]} and Hindemith^{[12]} 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^{[7]} provides the example of the final tonic chord of some popular music being traditionally analyzable as a "submediant sixfive 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 intervalclass integer to distinguish the interval. Thus the diminishedseventh 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.^{[13]} Alternative interval naming conventions
As shown below, some of the above mentioned intervals have alternative names, and some of them take a specific alternative name in Pythagorean tuning, fivelimit tuning, or meantone temperament tuning systems such as quartercomma meantone. Notice that ditone and semiditone are specific for Pythagorean tuning, while tone and tritone are used generically for all tuning systems. Interestingly, despite its name, a semiditone (3 semitones, or about 300 cents) can hardly be viewed as half of a ditone (4 semitones, or about 400 cents). All the intervals with prefix sesqui are justly tuned, and their frequency ratio, shown in the table, is a superparticular number (or epimoric ratio). The same is true for the octave. The diminished second is a comma, but some commas are not diminished seconds. For intance, the Pythagorean comma (531441:524288) is the opposite of a diminished second. 5limit tuning defines four kinds of comma, three of which meet the definition of diminished second, and hence are listed in the table below. The fourth one, called syntonic comma (81:80) can neither be regarded as a diminished second, nor as its opposite. See here for further details. 