The Old High German speaking area around 950.
During the Migration Period, the High German-speaking tribes settled in what became Alamannia, the Duchy of Bavaria and the Kingdom of the Lombards. At the same time the Franconian-speaking tribes settled the area between those two rivers before crossing the Rhine to conquer Northern Gaul, where, under the Merovingians, they created the Frankish kingdom, Francia, which eventually stretched down to the Loire.
Old High German comprises the dialects of these groups which underwent the Second Sound Shift during the 6th Century, namely all of Elbe Germanic and most of the Weser-Rhine Germanic dialects.
The Franks in the western part of Francia (Neustria and western Austrasia) gradually adopted Gallo-Romance by the beginning of the OHG period, with the linguistic boundary later stabilised approximately along the course of the Meuse and Moselle in the east, and the northern boundary probably a little further south than the current boundary between French and Flemish. North of this line, the Franks retained their language, but it was not affected by the Second Sound Shift, which thus separated their Low Franconian variety (the ancestor of Dutch) from the more easterly Franconian dialects which formed part of Old High German.
The Saxons and the Frisians along the shores of North Sea were likewise not affected by the Second Sound Shift and a bundle of isoglosses in a similar location to the modern Benrath line marked the Northern limit of the sound shift and separated the dialect of the Franks from Old Saxon.
In the south, the Lombards, who had settled in Northern Italy, maintained their dialect until their conquest by Charlemagne in 774. After this the Germanic-speaking population, who were by then almost certainly bilingual, gradually switched to the Romance language of the native population, so that Langobardic had died out by the end of the OHG period.
At the beginning of the period, no Germanic language was spoken east of a line from Kieler Förde to the rivers Elbe and Saale, earlier Germanic speakers in the Northern part of the area having been displaced by the Slavs. This area did not become German-speaking again until the German eastward expansion ("Ostkolonisation") of the early 12th century, though there was some attempt at conquest and missionary work under the Ottonians.
The Alemannic polity was conquered by Clovis I in 496, and in the last twenty years of the 8th century Charlemagne subdued the Saxons, the Frisians, the Bavarians, and the Lombards, bringing all continental Germanic-speaking peoples under Frankish rule. While this led to some degree of Frankish linguistic influence, the language of both the administration and the Church was Latin, and this unification did not therefore lead to any development of a supra-regional variety of Frankish nor a standardized Old High German; the individual dialects retained their identity.