“The Proto-Germanic language developed in southern Scandinavia (Denmark, south Sweden and southern Norway), the Urheimat (original home) of the Germanic tribes.” [in other words, Denmark]
Proto-Germanic developed from pre-Proto-Germanic into three branches during the first half of the first millennium of the Common Era: 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. 90 CE).
Map of the pre-Roman Iron Age in Northern Europe showing cultures associated with Proto-Germanic, c. 500 BC. The red shows the area of the preceding Nordic Bronze Age in Scandinavia; the magenta-colored area towards the south represents the Jastorf culture of the North German Plain.