for more intimate relationships (see T-V distinction). Otto Jahn, in his 1856 Mozart biography, reported that Haydn and Mozart used the informal du forms in conversation, an unusual practice at the time for two people of such different ages, hence evidence for a close friendship.
Haydn and Mozart reportedly used informal conversational styles, specifically the "you" form (du), which was unusual given their significant age gap. This behavior is highlighted as proof of their strong bond.