é, è, i and in are articulated with the lips open and the back part of the mouth gradually closed.
a, au, ou and on are articulated with the back of the mouth open and the lips gradually closed.
e, eu, u and un are articulated with the back of the mouth and the lips uniformly closed.
The voice takes different names, according to the different sounds in each family of vowels: the chest-voice, the medium voice and the head-voice.
These names imply no change in the sort of voice, but a change in the manner of emission. The head, medium or chest-voice, indicates only variety in the emission of vowels, and may be applied to the high as well as the deep and medium voice. Thus the deep voice may produce sounds in the head-voice, as well as in the medium and chest voices.
The head-voice is produced by lowering the larynx, and at the same time raising the uvula. In swallowing, the larynx rises by the elevation of the uvula, without which elevation there can be no head-tones.