A Dictionary of Music and Musicians/Third
THIRD. One of the most important intervals in modern music, since, by one or other of its principal forms, it supplies the means of definition in all the most characteristic chords. Three forms are met with in modern music—major, minor, and diminished. The first of these occurs most characteristically in the major scale between the Tonic and the Mediant as between C and E in the key of C (a). It is also an important factor in the Dominant chord, whether in the major or minor mode—as between G and B in the Dominant of the key of C (b). The minor third occurs most characteristically in the minor scale as the converse to the principal major third in the major scale; that is, between Tonic and Mediant; as C and E♭ in C minor (c). It also makes its appearance characteristically in the chord of the subdominant—as F–A♭ in C minor (d);
but both this minor third and the major third of the dominant chord are sometimes supplanted by major and minor thirds respectively for the convenience of melodic progression in the minor mode. In all fundamental discords, such as the Dominant seventh and Dominant major and minor ninths, the first interval from the root-note in the original position of the chord is a major third.
The major third is well represented in the series of partial tones or harmonics, by the tone which comes fourth in order, and stands in the second octave from the prime tone or generator.
The ratio of the sounds of the major third is 4:5, and that of the minor third 5:6. Thirds were not accepted by the ancients as consonances, and when they began to come into use in the early middle ages as so-called imperfect consonances the major third used was that commonly known as the Pythagorean third, which is arrived at by taking four fifths from the lower note. The ratio of this interval is 64:81, and it is therefore considerably sharper than the just or natural third; while the major third of equal temperament generally used in modern music lies between the two, but a little nearer to the Pythagorean third.
The resultant tones of thirds are strong. That of the major third is two octaves lower than the lowest of the two notes, and that of the minor third two octaves and a major third.
Diminished thirds are rough dissonances; they occur in modern music as the inversions of augmented sixths, as F♯—A♭ (e); and their ratio is 225:256. They are of powerful effect, but are sparingly used by great masters of the art. They rarely appear in the position of actual thirds, but more commonly in the extended position as diminished tenths.