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A major third is the larger of two commonly occurring musical intervals that span three diatonic scale degrees. The prefix 'major' identifies it as being the larger of the two (by one semitone); its smaller counterpart being a minor third. The major third is abbreviated as M3 and its inversion is the minor sixth. The major scale is so named because of the presence of this interval between its tonic and mediant (1st and 3rd) scale degrees. Major chords too take their name from the presence of this interval built on the chord's root (provided that the interval of a perfect fifth from the root is also present or implied). A major third in just intonation most often corresponds to a pitch ratio of 5:4 or 1.25:1, or various other ratios, while in an equal tempered tuning, a major third is equal to four semitones, a ratio of 1:24/12 (approximately 1.259), or 400 cents, 13.686 cents larger. The older concept of a ditone (two major seconds=major third) is the pythagorean ratio 81:64 (1.2656= 1.1252, two major seconds). The major third is classed as an imperfect consonance and is considered one of the most consonant intervals after the unison, octave, perfect fifth, and perfect fourth. In the common practice period, thirds were considered interesting and dynamic consonances along with their inverses the sixths, but in medieval times they were considered dissonances unusable in a stable final sonority.
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