Old High German language
This is the first independent step of the German language. After Common Germanic was divided into several separate tongues, people of the Early German Empire picked up the branch which was called Old High German, opposed to other branches: Old Low German or Old Saxon and Old Norse spoken in Scandinavia.

Old High German was influenced strongly by Latin, its literature and vocabulary. This was the first difference occurred between High and Low German. But internal processes were much more important. First of all, unvoiced fricatives p, t, k became unvoiced spirants pf, ss, ch, while they were preserved in Saxon. So Low German water means High German Wasser. The narrow long vowels became diphthongs in High German: e.g.  îs > eis (ice). Vice versa, short vowels were prolongated in open syllables: name > náme.

Old High German was productive and developed very quickly, already in the 12th century its morphology changed a lot. So the Middle High German epoch began, but that is already the history of the modern German language.