Major third
Four semitones, ratio 5:4. It makes a triad major.
What it means
The major third comprises four semitones; its pure frequency ratio is 5:4, and it appears comparatively low in the harmonic series. Its musical significance lies in the fact that it decides the mode: a triad with a major third above the root is major, one with a minor third minor. A single semitone separates the two.
Musical use
As an imperfect consonance the third may be led in parallel and thereby forms the basis of simple polyphony — the texture of melody and lower third is widespread worldwide. Historically this was not always so: the early Middle Ages knew only octave, fifth, and fourth as full-fledged sonorities, and the third established itself only in the fourteenth and fifteenth centuries. With it the sound of European music changed fundamentally, for only the sound of the third makes the triad the basic building block.
For intonation the major third is especially sensitive. In equal temperament it is noticeably wider than the pure one; ensembles without a keyboard instrument therefore take it lower so that the major chord becomes calm. In meantone tuning the purity of the thirds even became the yardstick for which the fifths were narrowed.
The major third is to be distinguished from the diminished fourth, which is the same in sound but different in spelling and function.
In Pythagorean tuning the major third is distinctly larger than 5:4 and sounds rough, which explains the medieval classification as an imperfect consonance. The meantone tuning of the Renaissance kept the major thirds pure, at the expense of other intervals. In the equal-tempered system the major third is somewhat larger than pure.
Performance practice
Take the major third in the major chord lower than the piano plays it, and listen for the disappearance of the beats — this is the most effective single intervention in the intonation of an ensemble. With parallel thirds, match attack and length so that they act as one sound. And practise hearing the difference between major and minor third deliberately; recognizing major and minor rests on it.