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When speaking of notes in the chromatic scale, it is normal to use names such as "major third", "perfect fifth" and "augmented fourth". When speaking of other scales, however, or when talking about two pitches without the context of a scale, such names are often meaningless. Some such intervals have names of their own.
It is also possible to measure the size of the interval between any two notes by using the logarithmic measure of cents. 1200 cents are equal to an octave, and an equally tempered semitone is equal to 100 cents.
Basic intervals: whole tone and semitone A semitone, (also called half tone or half step), is the smallest interval in Western musical tuning. It corresponds to the the difference between two adjacent keys on a piano, or between two frets on a guitar.
In terms of pitch, it is equal to a frequency ratio of 21/12 in equal temperament, 256/243 in Pythagorean tuning, (3/2)1/7 in perfect fifth tuning, and various rational numbers, such as 16/15, in just intonation.
A whole tone (also called tone or whole step) is equal to two semitones in equal temparament, where it is 21/6. It is 9/8 in Pythagorean tuning, (9/4)1/7 in fifth tuning, and either 9/8 or 10/9 in just intonation, depending on the note in the scale.
Intervals in the chromatic scale Intervals are named after the number of notes they span in the diatonic scale. The names are inclusive of the two notes being considered; for example the interval between a C and a G is a fifth (C,D,E,F,G is a distance of 5 notes). In addition to the number of tones between notes, the nature of the interval can also be described. In a major scale, intervals starting from the tonic can be perfect or major. A Unison is the interval between a note and itself (meaning normally just one note heard.)
In minor scales the minor interval is introduced:
suspensions.) Concordant intervals include all perfect intervals (4ths, 5ths, 8ves) and a few imperfect intervals, namely major and minor 3rds and 6ths; all other intervals are called discordant intervals, including 2nds, 7ths, and augmented or diminished notes.
Modified intervals often have more than one name. For example, a minor 7th can also be written as an augmented 6th. These are called enharmonic intervals. Typically names with 'minor' or 'major' in them are preferred, so the more correct way to write the interval given in the example is 'minor 7th.' The primary exception to this being in the case of a diminished 7th, which has a specific function within a full diminished 7th chord.
The interval of augmentation and diminishment is a semitone only in modern equal temperament, where a semitone is exactly half a tone. More generally it is a tone less a semitone. An interval is augmented by changing one its semitones into a tone. Also this means that an augmented 6th is not generally equal to a minor 7th. This is only true in modern music.
Other intervals There are also a number of intervals not found in the chromatic scale which have names of their own. These intervals describe small discrepencies between notes tuned according to just intonation:
A number of cultures around the world who do not use the chromatic scale have their own names for intervals found in their music.
For the mathematical use of the word "interval", see interval (mathematics).