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Inmusic theory, athird is amusical interval encompassing threestaff positions (seeInterval number for more details), and themajor third (Play^ⓘ) is a third spanning fourhalf steps or twowhole steps.^[1] Along with theminor third, the major third is one of two commonly occurring thirds. It is described asmajor because it is the larger interval of the two: The major third spans four semitones, whereas theminor third only spans 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.

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As Beward and Saker explain:

The intervals from thetonic (keynote) in an upward direction to the second, to the third, to the sixth, and to the seventhscale degrees of a major scale are called "major".^[2]

Diminished andaugmented thirds are shown on themusical staff the same number of lines and spaces apart, but contain a different number of semitones in pitch (two and five).

The major third may be derived from theharmonic series as the interval between the fourth and fifth harmonics. Themajor scale is so named because of the presence of this interval between itstonic andmediant (1st and 3rd)scale degrees. Themajor chord also takes its name from the presence of this interval built on the chord'sroot (provided that the interval of aperfect fifth from the root is also present).

A major third is slightly different in differentmusical tunings: Injust intonation it corresponds to a pitch ratio of 5:4, or⁠ 5 / 4⁠ (play^ⓘ) (fifth harmonic in relation to the fourth) or 386.31 cents; in12 tone equal temperament, a major third is equal to foursemitones, a ratio of 2^1/3:1 (about 1.2599) or 400 cents, 13.69 cents wider than the 5:4 ratio. The older concept of a "ditone" (two 9:8 major seconds) made a dissonant, wide major third with the ratio 81:64 (about 1.2656) or 408 cents (play^ⓘ), about22 cents sharp from the harmonic ratio of 5:4 . Theseptimal major third is 9:7 (435 cents), theundecimal major third is 14:11 (418 cents), and thetridecimal major third is 13:10 (452 cents).

In 12 tone equal temperament(12 TET) three major thirds in a row are equal to an octave. For example, A^♭ to C, C to E, and E to G^♯ (in12 TET, the differently written notes G^♯ and A^♭ both represent the same pitch, butnot in most othertuning systems). This is sometimes called the "circle of thirds". In just intonation, however, three 5:4 major third, the 125thsubharmonic, is less than an octave. For example, three 5:4 major thirds from C is B^♯ (C to E, to G^♯, to B^♯) (⁠  B^♯ / C⁠${\displaystyle =\frac{5^3}{2^6}=\frac{125}{64}}$). The difference between this just-tuned B^♯ and C, like the interval between G^♯ and A^♭, is called the "enharmonicdiesis", about 41 cents, or about twocommas (theinversion of the interval⁠ 125 / 64⁠:${\displaystyle \frac{128}{125}=\frac{2^7}{5^3}}$ (play^ⓘ)).

The major third is classed as animperfect consonance and is considered one of the most consonant intervals after theunison,octave,perfect fifth, andperfect fourth. In thecommon practice period, thirds were considered interesting and dynamic consonances along with theirinverses the sixths, but inmedieval times they were considered dissonances unusable in a stable final sonority.

In equal temperament, adiminished fourth isenharmonically equivalent to a major third (that is, it spans the same number of semitones). For example, B–D^♯ is a major third; but if the same pitches are spelled as the notes B and E^♭, then the interval they represent is instead adiminished fourth. The difference inpitch is erased in12 tone equal temperament, where the distinction is only nominal, but the difference between a major third and a diminished fourth is significant in almost all othermusical tuning systems. B–E^♭ occurs in the C harmonic minor scale.

The major third is used inguitar tunings. For thestandard tuning, only the interval between the 3rd and 2nd strings (G to B, respectively) is a major third; each of the intervals between the other pairs of consecutive strings is aperfect fourth. In analternative tuning, themajor-thirds tuning,each of the intervals are major thirds.

• Minor thirds:

C-E♭ Strings playing the note C then E♭ Problems playing this file? Seemedia help.

C-A Strings playing the note C then A Problems playing this file? Seemedia help.

• Major thirds

Major third (equal temperament) The file playsmiddle C, followed by E (a tone 400 cents sharper than C), followed by both tones together. Problems playing this file? Seemedia help. C-E Strings playing the note C then E Problems playing this file? Seemedia help.

C-A♭ Strings playing the note C then A♭ Problems playing this file? Seemedia help.

• Decade (log scale), compound just major third^{[clarification needed]}
• Ear training
• Ladder of thirds / chain of thirds
• List of meantone intervals
• Circle of thirds

• Thirds (music)
• Major intervals

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