Vulgar Latin is often confused with Proto-Romance. Proto-Romance is a proto-language, i.e. the latest stage common to all of the Romance languages. Because some of the less familiar Romance languages branched off early from the others (Sardinian in particular, followed by Romanian and related Eastern Romance languages), it is also common to reconstruct later stages: e.g. Proto Continental Romance (after Sardinian branched off); Proto Italo-Western Romance (after Sardinian and Romanian branched off); and Proto Western Romance (after the branching-off of Sardinian, Romanian, and the central and southern Italian languages, including standard Italian).
Proto-Romance and the other proto-languages are theoretical, unitary linguistic constructions. Vulgar Latin, on the other hand, is the actual speech of the common people during the late Roman Empire. As a result, it is not simply theoretical but actually attested (if thinly), and is not unitary, with differences over both time and space. Hence, it is possible to speak of, for example, the loss of initial /j/ in unstressed syllables in the Vulgar Latin of Cantabria (the area in northern Spain that gave birth to modern Spanish), while it is inaccurate to speak of a similar change in the "Proto-Romance of Cantabria".
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