[…] 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), Pag. 64