Standard Italian began to be developed in the 13th and 14th centuries as a literary dialect. At first basically a Florentine dialect, stripped of local peculiarities, it has since acquired some characteristics of the dialect of Rome in particular and has always been heavily influenced by Latin. It overlies a wide variety of dialects, which are sometimes considered to represent a fundamental differentiation between northern and southern Italy that dates from Roman times.
Today, however, these variant dialects form a continuum of intelligibility, although geographically distant dialects may be radically different. The northern dialects include what are often called the Gallo-Italian dialects (Piedmontese, Lombard, Ligurian, Emilian-Romagnol); as the influence of a Celtic (Gaulish) substratum is discernible, some linguists consider them separate languages pertaining to the Gallo-Romance Subgroup. The other northern group of dialects, spoken in northeastern Italy, is called Venetan (including Venetian, Veronese, Trevisan, and Paduan dialects, etc.). Istrian, which is spoken on the peninsula now divided between Croatia and Slovenia, with a tiny portion belonging to Italy, is sometimes considered yet another northern Italian dialect, or an independent language of the Balkano-Romance Subgroup. The Tuscan dialects (including those of Corsica) are often held to form a linguistic group of their own, while in the south and east three broad dialect areas are grouped loosely together: (1) the dialects of the Marche (Marchigiano), Umbria, and Rome; (2) Abruzzian, Apulian, Neapolitan, Campanian, and Lucanian; and (3) Calabrian, Otrantan, and Sicilian, which are believed by some to be influenced by the Greek once spoken there (which still survives in isolated pockets on the extreme southern portion of the peninsula).
In modern Italy dialects are still the primary spoken idiom, though the standard Italian is virtually the only written language. Speakers of an Italian dialect, even one as superficially different as Sicilian, can with effort understand standard Italian, however, and can even learn it by such means as listening to radio programs. For most Italians their first contact with the standard language comes in primary school, in which until recently it was the only dialect used; standard Italian is virtually the only dialect of culture in modern Italy, and with immigration from the south to the industrial north it is becoming increasingly the language of intercommunication.