Upon this openest tone [a] various modifications are produced by narrowing the oral cavity, at different points and to different degrees […] the cavity may be so narrowed, at one and another point; that the friction of the breath, as driven out through the aperture, forms the conspicuous element in the audible product; this, then, is a sound of very different character, a fricative consonant.
- Whitney (1875), Pag. 61