Next to the mutes in regard to degree of closure are the class of so-called “fricatives,” defined […] as containing a rustling or friction of the breath through a narrowed aperture as their main element […] it is found convenient to divide the fricatives into two sub-classes: ‘s’ and ‘z’ have a peculiar quality which we call sibilant or hissing; and the same is shared by the ‘sh’ and the ‘zh’ (in ‘azure’, ‘vision’) sounds; which are produced farther back upon the roof of the mouth, or in a more palatal position. These two pairs, accordingly, we set by themselves, as lingual and palatal “sibilants.”
- Whitney (1875), a pag.64

We have seen that in the classes of mutes and fricatives the sounds go in pairs, one produced by mere breath, the other by intonated breath, forced through the same position of the organs […].
- Whitney (1875), a pag.66