cahn

I'm currently brushing up my Fritz knowledge by reading through a biography I hadn't read before, and though I'm only at the second Silesian war, the author has already quoted letters to several recipients where he's using du/tu - Wilhelmine (that I knew; from my recollection of their correspondance, they keep switching between tu and vous), Keyserling and in one case (the "You're my heir if I kick it in this battle" letter) August Wilhelm, though it's strictly vous with August W. on other occasions.

In any case: 18th century nobility throughout Europe was Vous/Ihr/Sie all the way, at least in writing, when talking to their nearest and dearest; Maria Theresia wrote "Vous" to Marie Antoinette throughout, and so did Joseph, though several anecdotes have it they used "du" when actually talking to her, for example.

A generation post Fritz, the explosion of German literature he so disdainfully ignored contributed to changing that. All the Sturm und Drang poets were really into calling each other Du. In literature, you have Carlos asking (very urgently) Posa to call him Du in Schiller's Don Carlos (I don't know how much of this passage made it into translation, cahn at their first reunion in the play because he can't bear the distance of Sie/Ihr. It's a sign of their different social standing that Faust in their second encounter says Du to Gretchen already while she still says Ihr to him. (And a sign they've had sex when she switches to Du as well.) And so forth.