Examples of simple musical intervals.
In music, the inclusive distance between one tone and another, whether sounded successively (melodic interval) or simultaneously (harmonic interval). In Western music, intervals are generally named according to the number of scale-steps within a given key
that they embrace; thus, the ascent from C to G (C–D–E–F–G) is called a fifth because the interval embraces five scale degrees. There are four perfect intervals: prime, or unison; octave; fourth; and fifth. The other intervals (seconds, thirds, sixths, sevenths) have major and minor forms that differ in size by a half step (semitone). Both perfect and major intervals may be augmented, or enlarged by a half tone. Perfect and minor intervals may be diminished, or narrowed by a half tone.
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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.