Vennemann presents a plausible alternative to Grimm's succession of Germanic and High German sound shifts. As part of his argumentation, Vennemann gives a systematic revision of the Proto-Germanic obstruent inventory which he reconstructs internally from Germanic data. He also looks beyond the Germanic data to posit a Paleo-Germanic obstruent system. While revising Grimm's traditional theory, Vennemann also reformulates Verner's Shift and how it would apply to his alternative glottalic approach.
Smith, L. C. (1996). Vennemann's bifurcation theory of the Germanic and German consonant shifts. Calgary Working Papers in Linguistics, 18(Winter), 67-76.