J. Schuyler Long,
The Sign Language
(1918)
“ One may express the idea of quietness by simply laying the fingers on the lips. But when the same finger is thrown violently against the lips with a rebuke expressed in the face, it becomes "keep still," while with an impudent or threatening look it may express "shut up." The facial grimaces, however, and the "mouthing" affected by some, are in no way a part of the sign and the habit is to be strongly condemned. ”

