In classical music, a third is a musical interval encompassing three staff positions, and the major third is a third spanning four semitones. Along with the minor third, the major third is one of two commonly occurring thirds. It is qualified as major because it is the larger of the two: the major third spans four semitones, the minor third three. For example, the interval from C to E is a major third, as the note E lies four semitones above C, and there are three staff positions from C to E. Diminished and augmented thirds span the same number of staff positions, but consist of a different number of semitones. The major third may be derived from the harmonic series as the interval between the fourth and fifth harmonics. The major scale is so named because of the presence of this interval between its tonic and mediantscale degrees. The major chord also takes its name from the presence of this interval built on the chord's root. A major third is slightly different in different musical tunings: in just intonation corresponds to a pitch ratio of 5:4 or 386.31 cents; in equal temperament, a major third is equal to four semitones, a ratio of 21/3:1 or 400 cents, 13.69 cents wider than the 5:4 ratio. The older concept of a ditone made a dissonantly wide major third with the ratio 81:64 . The septimal major third is 9:7, the undecimal major third is 14:11, and the tridecimal major third is 13:10. A helpful way to recognize a major third is to hum the first two notes of "Kumbaya" or of "When the Saints Go Marching In". A descending major third is heard at the starts of "Goodnight, Ladies" and "Swing Low, Sweet Chariot". In equal temperament three major thirds in a row are equal to an octave. This is sometimes called the "circle of thirds". In just intonation, however, three 5:4 major third, the 125th subharmonic, is less than an octave. For example, three 5:4 major thirds from C is B . The difference between this just-tuned B and C, like that between G and A, is called the "enharmonic diesis", about 41 cents ). 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. A diminished fourth is enharmonically equivalent to a major third. For example, B–D is a major third; but if the same pitches are spelledB and E, the interval is instead a diminished fourth. B–E occurs in the C harmonic minor scale. The major third is used in guitar tunings. For the standard tuning, only the interval between the 3rd and 2nd strings is a major third; each of the intervals between the other pairs of consecutive strings is a perfect fourth. In an alternative tuning, the major-thirds tuning, each of the intervals are major thirds.
