Tuscan doesn't measure up: Dante on his native dialect in the De Vulgari

Dante's De Vulgari Eloquentia is his treatise composed in Latin that defended the use of Italian in poetry and prose. In that time, writing for posterity in the spoken tongue of the living was not a self-evident thing to do, especially as regards works of literary aspiration.

The treatise was never completed - he originally envisioned four books, with the last to treat of comedy. We don't know why he dropped the project, but his sole mention of the comic comes in Book II.4, in contrast with the tragic mode:

By 'tragic' I mean the higher style, by 'comic' the lower, and by 'elegiac' that of the unhappy. If it seems appropriate to use the tragic style, then the illustrious vernacular must be employed, and so you will need to bind together a canzone. If, on the other hand, the comic style is called for, then sometimes the middle level of the vernacular can be used, and sometimes the lowly; and I shall explain the distinction in Book Four.

In an earlier section (Book I.13) we get a bit of the sardonic sensibility of the poet, writing about his own Tuscan dialect in the mouths of his fellow men:

After this, we come to the Tuscans, who, rendered senseless by some aberration of their own, seem to lay claim to the honour of possessing the illustrious vernacular. And it is not only the common people who lose their heads in this fashion, for we find that a number of famous men have believed as much: like Guittone d'Arezzo, who never even aimed at a vernacular worthy of the court, or Bonagiunta da Lucca, or Gallo of Pisa, or Mino Mocato of Siena, or Brunetto the Florentine, all of whose poetry, if there were space to study it closely here, we would find to be fitted not for a court but at best for a city council. Now, since the Tuscans are the most notorious victims of this mental intoxication, it seems both appropriate and useful to examine the vernaculars of the cities of Tuscany one by one, and thus to burst the bubble of their pride. When the Florentines speak, they say things like: 'Manichiamo, introcque che noi non facciamo altro' [Let's eat, since there's nothing else to do]. The Pisans: 'Bene andonno li fatti de Fiorensa per Pisa' [The business at Florence went well for Pisa]. The people of Lucca: 'Fo voto a Dio ke ingrassarra eie lo comuno de Lucca' [I swear to God, the city of Lucca is really in the pink]. The Sienese: 'Onche renegata avess'io Siena. Chée chesto?' [If only I'd left Siena for good! What's up now?]. The people of Arezzo: 'Vuo' tu venire ovelle?' [Do you want to go somewhere?]. I have no intention of dealing with Perugia, Orvieto, Viterbo, or Città di Castello, because of their inhabitants' affinity with the Romans and the people of Spoleto. However, though almost all Tuscans are steeped in their own foul jargon, there are a few, I feel, who have understood the excellence of the vernacular: these include Guido, Lapo, and one other, all from Florence, and Cino, from Pistoia, whom I place unworthily here at the end, moved by a consideration that is far from unworthy. Therefore, if we study the languages spoken in Tuscany, and if we think what kind of distinguished individuals have avoided the use of their own, there can be no doubt that the vernacular we seek is something other than that which the people of Tuscany can attain. If there is anyone who thinks that what I have just said about the Tuscans could not be applied to the Genoese, let him consider only that if, through forgetfulness, the people of Genoa lost the use of the letter z, they would either have to fall silent for ever or invent a new language for themselves. For z forms the greater part of their vernacular, and it is, of course, a letter that cannot be pronounced without considerable harshness.

Ultimately, Dante pointed to a form of Italian found in no specific city or province, but rather cultivated by the best authors in their poetry and prose:

This is the language used by the illustrious authors who have written vernacular poetry in Italy, whether they came from Sicily, Apulia, Tuscany Romagna, Lombardy, or either of the Marches. And since my intention, as I promised at the beginning of this work, is to teach a theory of the effective use of the vernacular, I have begun with this form of it, as being the most excellent;