"Old High German (OHG, German: Althochdeutsch, German abbr. Ahd.) is the earliest stage of the German language, conventionally covering the period from around 750 to 1050.There ...
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is no standardised or supra-regional form of German at this period, and Old High German is an umbrella term for the group of continental West Germanic dialects which underwent the set of consonantal changes called the Second Sound Shift.
At the start of this period, the main dialect areas belonged to largely independent tribal kingdoms, but by 788 the conquests of Charlemagne had brought all OHG dialect areas into a single polity. The period also saw the development of a stable linguistic border between French and German." - (en.wikipedia.org 25.08.2020)