Der Rosenkavalier
The language used by Hofmannsthal changes according to who is speaking.  The members of the nobility speak in very elegant language which is often old-fashioned and always very polite.  When good friends speak to one another they often use the familiar (du) (“you” when used between very close friends).  For example: Octavian and the Marschallin often use the familiar "you" but sometimes use (Sie) when they are being more formal.