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During the Middle Ages, the established written language in Europe was Latin, though the great majority of people were illiterate, and only a handful were well versed in the language. In the Italian peninsula, as in most of Europe, most would instead speak a local vernacular. These dialects (as they are commonly referred to) were born from Vulgar Latin over the course of centuries, evolving naturally unaffected by formal standards and teachings. They are not in any sense "dialects of" standard Italian, that itself started off being one of these local tongues, but sister languages of Italian. Mutual intelligibility with Italian varies widely, as it does with Romance languages in general. The Romance dialects of Italy can differ greatly from Italian at all levels (phonology, morphology, syntax, lexicon, pragmatics) and are classified typologically as distinct languages. 
The standard Italian language has a poetic and literary origin in the writings of Tuscan writers of the 12th century, and, even though the grammar and core lexicon are basically unchanged from those used in Florence in the 13th century, the modern standard of the language was largely shaped by relatively recent events. However, Romance vernacular as language spoken in the Appenine peninsula has a longer history. In fact, the earliest surviving texts that can definitely be called vernacular (as distinct from its predecessor Vulgar Latin) are legal formulae known as the Placiti Cassinesi from the Province of Benevento that date from 960–963, although the Veronese Riddle, probably from the 8th or early 9th century, contains a late form of Vulgar Latin that can be seen as a very early sample of a vernacular dialect of Italy.
The language that came to be thought of as Italian developed in central Tuscany and was first formalized in the early 14th century through the works of Tuscan writer Dante Alighieri, written in his native Florentine. Dante's epic poems, known collectively as the Commedia, to which another Tuscan poet Giovanni Boccaccio later affixed the title Divina, were read throughout the peninsula and his written dialect became the "canonical standard" that all educated Italians could understand. Dante is still credited with standardizing the Italian language. In addition to the widespread exposure gained through literature, the Florentine dialect also gained prestige due to the political and cultural significance of Florence at the time and the fact that it was linguistically an intermediate between the northern and the southern Italian dialects.:22 Thus the dialect of Florence became the basis for what would become the official language of Italy.
Italian was progressively made an official language of most of the Italian states predating unification, slowly replacing Latin, even when ruled by foreign powers (like Spain in the Kingdom of Naples, or Austria in the Kingdom of Lombardy-Venetia), even though the masses kept speaking primarily their local vernaculars. Italian was also one of the many recognised languages in the Austro-Hungarian Empire.
Italy has always had a distinctive dialect for each city because the cities, until recently, were thought of as city-states. Those dialects now have considerable variety. As Tuscan-derived Italian came to be used throughout Italy, features of local speech were naturally adopted, producing various versions of regional Italian. The most characteristic differences, for instance, between Roman Italian and Milanese Italian are the gemination of initial consonants and the pronunciation of stressed "e", and of "s" in some cases: e.g. va bene "all right": is pronounced [va ˈbːɛne] by a Roman (and by any standard Italian speaker), [va ˈbene] by a Milanese (and by any speaker whose native dialect lies to the north of the La Spezia–Rimini Line); a casa "at home" is [a ˈkːasa] for Roman and standard, [a ˈkaza] for Milanese and generally northern.
In contrast to the Gallo-Italic linguistic panorama of northern Italy, the Italo-Dalmatian Neapolitan and its related dialects were largely unaffected by the Franco-Occitan influences introduced to Italy mainly by bards from France during the Middle Ages, but after the Norman conquest of southern Italy, Sicily became the first Italian land to adopt Occitan lyric moods (and words) in poetry. Even in the case of Northern Italian languages, however, scholars are careful not to overstate the effects of outsiders on the natural indigenous developments of the languages.
The economic might and relatively advanced development of Tuscany at the time (Late Middle Ages) gave its language weight, though Venetian remained widespread in medieval Italian commercial life, and Ligurian (or Genoese) remained in use in maritime trade alongside the Mediterranean. The increasing political and cultural relevance of Florence during the periods of the rise of the Banco Medici, Humanism, and the Renaissance made its dialect, or rather a refined version of it, a standard in the arts.
The Renaissance era, known as il Rinascimento in Italian, was seen as a time of "rebirth", which is the literal meaning of both renaissance (from French) and rinascimento (Italian).
During this time, long-existing beliefs stemming from the teachings of the Roman Catholic Church began to be understood from new a perspectives as humanists—individuals who placed emphasis on the human body and its full potential—began to shift focus from the church to human beings themselves. Humanists began forming new beliefs in various forms: social, political, and intellectual. The ideals of the Renaissance were evident throughout the Protestant Reformation, which took place simultaneously with the Renaissance. The Protestant Reformation began with Martin Luther's rejection of the selling of indulgences by Johann Tetzel and other authorities within the Roman Catholic Church, resulting in Luther's eventual break-off from the Roman Catholic Church in the Diet of Worms. After Luther was excommunicated from the Roman Catholic Church, he founded what was then understood to be a sect of Catholicism, later referred to as Lutheranism. Luther's preaching in favor of faith and scripture rather than tradition led him to translate the Bible into many other languages, which would allow for people from all over Europe to read the Bible. Previously, the Bible was only written in Latin, but after the Bible was translated, it could be understood in many other languages, including Italian. The Italian language was able to spread even more with the help of Luther and the invention of the printing press by Johannes Gutenberg. The printing press facilitated the spread of Italian because it was able to rapidly produce texts, such as the Bible, and cut the costs of books which allowed for more people to have access to the translated Bible and new pieces of literature. The Roman Catholic Church was losing its control over the population, as it was not open to change, and there was an increasing number of reformers with differing beliefs.
was an influential figure in the development of the Italian language from the Tuscan dialect, as a literary medium, codifying the language for standard modern usage.
Italian became the language used in the courts of every state in the Italian peninsula. The rediscovery of Dante's De vulgari eloquentia and a renewed interest in linguistics in the 16th century, sparked a debate that raged throughout Italy concerning the criteria that should govern the establishment of a modern Italian literary and spoken language. This discussion, known as questione della lingua (i. e., the problem of the language), ran through the Italian culture until the end of the 19th century, often linked to the political debate on achieving a united Italian state. Renaissance scholars divided into three main factions:
A fourth faction claimed the best Italian was the one that the papal court adopted, which was a mix of Florentine and the dialect of Rome.
Eventually, Bembo's ideas prevailed, and the foundation of the Accademia della Crusca in Florence (1582–1583), the official legislative body of the Italian language led to publication of Agnolo Monosini's Latin tome Floris italicae linguae libri novem in 1604 followed by the first Italian dictionary in 1612.
The continual advancements in technology plays a crucial role in the diffusion of languages. After the invention of the printing press in the fifteen century, the number of printing presses in Italy grew rapidly and by the year 1500 reached a total of 56, the biggest number of printing presses in all of Europe. This allowed to produce more pieces of literature at a lower cost and as the dominant language, Italian spread.
An important event that helped the diffusion of Italian was the conquest and occupation of Italy by Napoleon in the early 19th century (who was himself of Italian-Corsican descent). This conquest propelled the unification of Italy some decades after and pushed the Italian language into a lingua franca used not only among clerks, nobility, and functionaries in the Italian courts but also by the bourgeoisie.
Italian literature's first modern novel, I Promessi Sposi (The Betrothed), by Alessandro Manzoni, further defined the standard by "rinsing" his Milanese "in the waters of the Arno" (Florence's river), as he states in the Preface to his 1840 edition.
After unification, a huge number of civil servants and soldiers recruited from all over the country introduced many more words and idioms from their home languages ("ciao" is derived from the Venetian word "s-cia[v]o" (slave), "panettone" comes from the Lombard word "panetton" etc.). Only 2.5% of Italy's population could speak the Italian standardized language properly when the nation was unified in 1861.