By its nature Vulgar Latin varied greatly by region and by time period, though several major divisions can be seen. Vulgar Latin dialects began to significantly diverge from Classical Latin in the third century during the classical period of the Roman Empire. Nevertheless, throughout the sixth century the most widely spoken dialects were still similar to and mostly mutually intelligible with Classical Latin.
The verb system seems to have remained virtually intact throughout the fifth century the transformation of the language, from structures we call Latin into structures we call Romance, lasted from the third or fourth century until the eighth; we could say Latin "died" in the first part of the eighth century; in Italy the first signs that people were aware of the difference between the everyday language they spoke and the written form is in the mid-tenth century. The period of most rapid change occurred from the second half of the seventh century. Until then the spoken and written form (though with many vulgar features) were regarded as one language.
The language called Proto-Romance developed during the governance of Germanic rulers. Similarly, in the Eastern Roman Empire Latin faded as the Court language in the course of the 5th century. The Vulgar Latin spoken in the Balkans north of Greece and southern Bulgaria became heavily influenced by Greek and Slavic and also became radically different from Classical Latin and from the proto-Romance of Western Europe. Vulgar Latin diverged into distinct languages beginning in the 9th century.
The term "common speech" (sermo vulgaris), which later became "Vulgar Latin", was used by inhabitants of the Roman Empire. Subsequently it became a technical term from Latin and Romance-language philology referring to the unwritten varieties of a Latinised language spoken mainly by Italo-Celtic populations governed by the Roman Republic and the Roman Empire.
Traces of their language appear in some inscriptions, such as graffiti or advertisements. The educated population mainly responsible for Classical Latin may also have spoken Vulgar Latin in certain contexts depending on their socioeconomic background. The term was first used improperly in that sense by the pioneers of Romance-language philology: François Juste Marie Raynouard (1761–1836) and Friedrich Christian Diez (1794–1876).
In the course of his studies on the lyrics of songs written by the troubadours of Provence, which had already been studied by Dante Alighieri and published in De vulgari eloquentia, Raynouard noticed that the Romance languages derived in part from lexical, morphological, and syntactic features that were Latin, but were not preferred in Classical Latin. He hypothesized an intermediate phase and identified it with the Romana lingua, a term that in countries speaking Romance languages meant "nothing more or less than the vulgar speech as opposed to literary or grammatical Latin."
Diez, the principal founder of Romance-language philology, impressed by the comparative methods of Jakob Grimm in Deutsche Grammatik, which came out in 1819 and was the first to use such methods in philology, decided to apply them to the Romance languages and discovered Raynouard's work, Grammaire comparée des langues de l'Europe latine dans leurs rapports avec la langue des troubadours, published in 1821. Describing himself as a pupil of Raynouard, he went on to expand the concept to all Romance languages, not just the speech of the troubadours, on a systematic basis, thereby becoming the originator of a new field of scholarly inquiry.