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).