Northwest Germanic is a proposed grouping of the Germanic languages, representing the current consensus among Germanic historical linguists. It does not challenge the late 19th-century tri-partite division of the Germanic dialects into North Germanic, West Germanic and East Germanic, but proposes additionally that North and West Germanic (i.e. all surviving Germanic languages today) remained as a subgroup after the southward migration of the East Germanic tribes, only splitting into North and West Germanic later. Whether this subgroup constituted a unified proto-language, or simply represents a group of dialects that remained in contact and close geographical proximity, is a matter of debate, but the formulation of Ringe and Taylor probably enjoys widespread support:

There is some evidence that North and West Germanic developed as a single language, Proto-Northwest Germanic, after East Germanic had begun to diverge. However, changes unproblematically datable to the PNWGmc period are few, suggesting that that period of linguistic unity did not last long. On the other hand, there are some indications that North and West Germanic remained in contact, exchanging and thus partly sharing further innovations, after they had begun to diverge, and perhaps even after West Germanic had itself begun to diversify.

— Don Ringe / AnnTaylor, The Development of Old English, p. 10

Though not yet using the term ′Northwest Germanic′, this grouping was proposed by Hans Kuhn in 1955/56 as an alternative to the older view of a Gotho-Nordic versus West Germanic division. Instead, Kuhn used the term ′Spätgemeingermanisch′ (Late Common Germanic):

In dem Zeitraum, da sich dieser [der ostgermanische] Ast vom großen Stamme [des Germanischen] löste, bildete sich die Sprache der Nord- und Westgermanen ebenso wie bisher noch fast ganz einheitlich fort. […]. Die Älteren urnordischen Inschriften enthalten noch kaum etwas, das sie als nordisch und nicht auch vorwestgermanisch verrät, aber dem Gotischen ist ihr Lautstand ferngerückt. Ich habe diese Stufe ‚Spätgemeingermanisch‘ genannt (Anz. 63, 8). Das Gotische wirkt ihm gegenüber in manchem fast wie vorgermanisch. Dies währte, von einzelnen Sonderungen, die erkennbar werden, abgesehn, bis um die Mitte des 1. Jahrtausends. Erst dann zerbrach diese Einheit.

During the period when this [East Germanic] branch broke away from the main [Germanic] Stamme [trunk], the language of the North and West Germanic peoples continued to develop in a manner that was, as before, almost entirely uniform. [...]. The older Proto-Norse inscriptions contain almost nothing that identifies them as Norse rather than Pre-West Germanic, yet their phonology has already diverged significantly from that of Gothic. I have termed this stage ‘Late Common Germanic’ (Anz. 63, 8). In comparison, Gothic appears in some respects almost pre-Germanic. This unity persisted, apart from certain peculiarities that become discernible, until around the middle of the 1st millennium. Only then did this unity break down.

— Hans Kuhn, Zur Gliederung der germanischen Sprachen [Apropos: The devision of Germanic langaugaes], p. 45 (= Kuhn 1969, 287)

Erst nach der Mitte des 1. Jahrtausends nach Christus, […], begann, soweit die Runeninschriften erkennen lassen, das Nordische sich mit eigenen Neuerungen spürbar vom Westgermanischen abzuheben.