The place and area where Old French was spoken natively roughly extended to the northern half of the Kingdom of France and its vassals (including parts of the Angevin Empire, which during the 12th century remained under Anglo-Norman rule), and the duchies of Upper and Lower Lorraine to the east (corresponding to modern north-eastern France and Belgian Wallonia), but the influence of Old French was much wider, as it was carried to England, Sicily and the Crusader states as the language of a feudal elite and of commerce.
The areal of Old French in contemporary terms corresponded to the northern parts of the Kingdom of France (including Anjou and Normandy, which in the 12th century were ruled by the Plantagenet kings of England), Upper Burgundy and the duchy of Lorraine. The Norman dialect was also spread to England and Ireland, and during the crusades, Old French was also spoken in the Kingdom of Sicily, and in the Principality of Antioch and the Kingdom of Jerusalem in the Levant.
As part of the emerging Gallo-Romance dialect continuum, the langues d'oïl were contrasted with the langue d'oc (the emerging Occitano-Romance group, at the time also called Provençal), adjacent to the Old French area in the south-west, and with the Gallo-Italic group to the south-east. The Franco-Provençal group developed in Upper Burgundy, sharing features with both French and Provençal; it may have begun to diverge from the langue d'oïl as early as the 9th century, and is attested as a distinct Gallo-Romance variety by the 12th century.
Dialects or variants of Old French included:
Some modern languages are derived from Old French dialects other than Classical French, which is based on the Île-de-France dialect. They include Angevin, Berrichon, Bourguignon-Morvandiau, Champenois, Franc-Comtois, Gallo, Lorrain, Norman, Picard, Poitevin, Saintongeais and Walloon.
Beginning with Plautus’s time (254–184 b.c.), Classical Latin’s phonological structure changed, eventually yielding Vulgar Latin, the common spoken language of the Western Roman Empire. This latter form differed strongly from its classical counterpart in phonology and morphology, as well as exhibiting differences in lexicon; it was the ancestor of the Romance languages, including Old French.
Some Gaulish words influenced Vulgar Latin and, through this, other Romance languages. For example, classical Latin equus was uniformly replaced in Vulgar Latin by caballus ‘nag, work horse’, derived from Gaulish caballos (cf. Welsh ceffyl, Breton kefel), giving Modern French cheval, Occitan caval (chaval), Catalan cavall, Spanish caballo, Portuguese cavalo, Italian cavallo, Romanian cal, and, by extension, English cavalry. An estimated 200 words of Gaulish etymology survive in modern French, for example chêne ‘oak tree’ and charrue ‘plough’.