Published, edited, images added & annotations/comments (in red) by Kenneth S. Doig
(at the above-referenced link, you can read the original documents with all its footnotes, citations & references)
In historical linguistics, the Germanic (Gmc) parent-language (GPL) includes the reconstructed languages in the Gmc group referred to as Pre-Germanic Indo-European (IE), (PreGmc), Early Proto-Germanic (EPGmc), and Late Proto-Germanic (LPGmc), spoken in the 2nd and 1st millennia BC.
(PIE = Proto-Indo-European)
As an identifiable neologism, the term appears to have been first used by Frans Van Coetsem in 1994. It also makes appearances in the works of Elzbieta Adamczyk, Jonathan Slocum and Winfred P. Lehmann.
Several historical-linguists have pointed towards the apparent material- and social-continuity connecting the cultures of the Nordic Bronze-Age (1800–500 BC) and the Pre-Roman Iron-Age (500 BC–1 AD) as having implications in regard to the stability and later development of the Germanic language-group.
The emerging consensus among scholars is that the first Gmc soundshift— long considered to be the defining mark in the development of PGmc—happened as late as 500 BC.
Research conducted over the past few decades displays a notable interest in exploring the linguistic and sociohistorical conditions under which this soundshift occurred, and often formulates theories and makes reconstructive-efforts regarding the periods immediately preceding Proto-Germanic as traditionally characterized.
The notion of the Germanic parent-language is thus used to encompass both the Pre-Proto-Germanic stage of development preceding the first Gmc soundshift (assumed to be contemporary with the Nordic Bronze-Age) and that stage traditionally identified as Proto-Germanic up to the beginning of the "common-era" (or Anno Domini).
The upper-boundary assigned to the Germanic parent-language is described as "dialectical Indo-European". In the works of both Van Coetsem and Voyles, attempts are made to reconstruct aspects of this stage of the language using a process the former refers to as inverted reconstruction; i.e., one using the data made available through the attested daughter-languages in light of and at times in preference to the results of the comparative reconstruction undertaken to arrive at PIE.
The results are not strictly standard in terms of traditional PIE-reconstruction, but they are instead presented as characteristic of the incipient predecessor to Early Proto-Germanic, hence the terms Pre-Germanic Indo-European (Voyles) or Pre-Proto-Germanic (Van Coetsem) for this stage.
The lower-boundary of the Germanic parent-language has been tentatively identified as that point in the development of the language which preceded permanent fragmentation and which produced the Germanic daughter-languages.
In his work entitled The Vocalism of the Germanic parent-language, Frans Van Coetsem lays out a broad set of phonological characteristics which he considers to be representative of the various stages encompassed by the Gmc parent-language:
Early Proto-Germanic: (1) *ā/*ă, *ō/*ŏ mergers; (2) dissolution of the syllabic-liquids and nasals; (3) the initiation of fricativization or the first consonant-shift (also known as Grimm's law or (die) Erste Lautverschiebung)
Late Proto-Germanic: (1) accent-modification in two stages: (a) intensification in dominance followed by Verner's law; (b) fixation on the first syllable: umlaut- (vowel-mutation) and accent-conditioned raising and lowering changes; reduction in non-accented position; (3) /*s/ → /*z/.
Other rules thought to have affected the Pre-Germanic stage include Cowgill's law, which describes the process of laryngeal-loss known to have occurred in most post-PIE (IE) dialects, and Osthoff's law, which describes rules for the shortening of long vowels, known to have applied in western dialects such as Greek, Latin, and Celtic, but not in Tocharian or Indo-Iranian.
Ringe (2006) suggests that it is likely that Osthoff's law also applied to Germanic, and that the loss of laryngeals such as *h2 must have preceded the application of Grimm's law.