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|PGmc, Common Germanic|
|Reconstruction of||Germanic languages|
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Proto-Germanic eventually developed from pre-Proto-Germanic into three Germanic branches during the fifth century BC to fifth century AD: West Germanic, East Germanic and North Germanic, which however remained in contact over a considerable time, especially the Ingvaeonic languages (including English), which arose from West Germanic dialects and remained in continued contact with North Germanic.
A defining feature of Proto-Germanic is the completion of the process described by Grimm's law, a set of sound changes that occurred between its status as a dialect of Proto-Indo-European and its gradual divergence into a separate language. As it is probable that the development of this sound shift spanned a considerable time (several centuries), Proto-Germanic cannot adequately be reconstructed as a simple node in a tree model but rather represents a phase of development that may span close to a thousand years. The end of the Common Germanic period is reached with the beginning of the Migration Period in the fourth century.
The alternative term "Germanic parent language" may be used to include a larger scope of linguistic developments, spanning the Nordic Bronze Age and Pre-Roman Iron Age in Northern Europe (second to first millennia BC) to include "Pre-Germanic" (PreGmc), "Early Proto Germanic" (EPGmc) and "Late Proto-Germanic" (LPGmc). While Proto-Germanic refers only to the reconstruction of the most recent common ancestor of Germanic languages, the Germanic parent language refers to the entire journey that the dialect of Proto-Indo-European that would become Proto-Germanic underwent through the millennia.
The Proto-Germanic language is not directly attested by any coherent surviving texts; it has been reconstructed using the comparative method. Fragmentary direct attestation exists of (late) Common Germanic in early runic inscriptions (specifically the second-century AD Vimose inscriptions and the second-century BC Negau helmet inscription), and in Roman Empire era transcriptions of individual words (notably in Tacitus' Germania, c. AD 90).