AI overview
I cannot write an accurate overview based solely on "year" as context, since "855" could refer to the year 855 CE in history, the number 855 in mathematics, or various other things. To provide a reliable, factual overview grounded only in the information given, I would need more specific context about what "855" refers to.
AI-generated from the Wikipedia summary — may contain errors.
Article
Emperor Lothair I divides Middle Francia between his three sons (Treaty of Prüm).
Year 855 (DCCCLV) was a common year starting on Tuesday of the Julian calendar.