Theoretically, an indefinite number of mute-closures are possible, all along the mouth, from the lips to as far back in the throat as the organs can be brought together, in practice, however, they are found to be prevailingly three: one in the front, made by lip against lip, the labial closure, giving ‘p’; one in the back of the mouth, made against the soft palate by the rear upper surface of the tongue, the palatal (or guttural) closure, giving ‘k’; and one intermediate between the other two, made by the point or front of the tongue against the roof of the mouth near the front teeth, the lingual (or dental) closure, giving ‘t’.
- Whitney (1875), Pag. 61-62