“In referring to his conception of the vernacular, in the opening chapter of his treatise Dante uses terms such as vulgaris eloquentia, locutio vulgarium gentium and vulgaris locutio, while reserving the term locutio secundaria for grammar.
We can probably take eloquentia as generically “ability to speak fluently.” Nevertheless, the text contains a series of distinctions, and these are probably not casual. In certain instances, Dante speaks of locutio, in others of ydioma, of lingua or of loquela.
He uses the term ydioma whenever he refers to the Hebrew language (I, iv, 1; I, vi, 1; I, vi, 7) and when he expresses his notion of the branching off of the various languages of the world–the Romance languages in particular. In vi, 6-7, in speaking of the confusion after Babel, Dante uses the term loquela.
In this same context, however, he uses ydioma for the languages of the confusion as well as for the Hebrew language which remained intact. He can speak of the loquela of the Genovese and of the Tuscans while, at the same time, using lingua both for Hebrew and for the Italian vernacular dialects.
It thus seems that the terms ydioma, lingua and loquela are all to be understood as meaning a tongue or a given language in the modern, Saussurian sense of langue.
Often locutio is used in this sense too. When he wishes to say that, after the destruction of Babel, the workers on the tower began to speak imperfect languages, he writes, “tanto rudius nunc barbariusque locuntur.” A few lines later, referring to the Hebrew language in its original state, he uses the phrase, “antiquissima locutione.” (I, vi, 6-8).
Nevertheless, although ydioma, lingua and loquela are “marked” forms (used only where langue in the Saussurian sense is meant), the term locutio seems to have another, more elastic sense.
It is used whenever the context seems to suggest either the activity of speaking, or the functioning of the linguistic faculty. Dante often uses locutio to mean the act of speaking: for example, he says of animal sounds that they cannot be construed as locutio, meaning by this that they do not qualify as proper linguistic activity. (I, ii, 6-7).
Dante also uses locutio every time that Adam addresses God.
These distinctions are clearest in the passage (I, iv, 1) where Dante asks himself “to what man was the faculty of speech [locutio] first given, and what he said at the beginning [quod primus locutus fuerit], and to whom, and where, and when, and in what language [sub quo ydiomate] were the first acts of linguistic behavior [primiloquium] emitted?”
I think I am justified here in giving primiloquium this sense of “first linguistic behavior” on the analogy of Dante’s use of the terms tristiloquium and turpiloquium to characterize the evil way of speaking of the Romans and the Florentines.
Umberto Eco, The Search for the Perfect Language, translated by James Fentress, Blackwell. Oxford, 1995, pp. 38-40.