The first chord sounded in Wagner's Tristan und Isolde: f-b-d#-g#. It was not a new harmonic entity – it can be described as an enharmonic respelling of a half-diminished seventh – it was just used in a completely novel way. Instead of resolving to Db7, the dominant of Gb major, or to Bb7, the dominant of Eb minor (the relative minor), it goes completely crazy and resolves to E7 and then to A minor. This upset a lot of uptight music theorists at the time and led to a lot of scholarly debate which can be summarized thusly: "He can't do that! It's against the rules!" "I think he should be able to write his own music however he wants to." (huffily) "Well, it's not common practise."