The inherent quality of tone is reinforced by the co-vibrations of the air in the resonance cavities, the greater fulness of the sound being caused by the increased quantity of air which is set into vibration. The slightest alteration in the shape of these cavities affects the quality of vocal tone by altering the direction and size of the air columns. There is for every tone an air column of a certain size which most powerfully reinforces that tone; and every resonance cavity answers to some particular note better than to others. Timbre in the voice depends largely upon the echoing and re-echoing of these resonance chambers; and it needs but little reflection to see that the shape given to the mouth in pronouncing speech sounds--more especially vowel sounds, with all their various shades--interferes more or less with the purity and quality of tone. Hence the necessity in singing for modifying vowel pronunciation to suit the various tones and pitches of the voice. Every shade of vowel has a certain pitch of its own which is best produced by certain positions of the mouth, tongue, and soft palate. It is, therefore, necessary, carefully to shape the mouth so that, on notes of different pitches, the configuration of the mould may be that which gives the best quality of the particular vowel tone. There must be an unimpeded passage for the voice from the larnyx to the lips, and this cannot be obtained if the same vowel shades are maintained in song as in speech. The vowels which require the greatest alteration in position of the mouth are A, E, and U; E being quite the most difficult, because, contrary to the opinion of some teachers who consider it the best for forward production of tone, it keeps the sound farther back in the throat than any other vowel, shutting it up and making the sound thin and poor. Diligent practice before a mirror is necessary in order to acquire the best position of the buccal resonance chamber; its attainment will well repay the trouble taken, for not only will the voice gain in timbre, in resonance, and in ease, but pronunciation will become pure and clear. The vowel "ah" is frequently chosen as the best one for vocalising, because in its pronunciation it is easy to put the mouth in a good position; and voices are trained on it exclusively, with the result that no other vowel, or vowel shade, is perfectly produced. Actual false intonation often arises from want of practice in adjusting the cavity of the mouth to that shape required for producing the best tone and resonance on the different notes; the absence of co-ordination between the fundamental tone and the overtones preventing perfect tune.