Lulu (opera)
Some of the characters have their own music which, in each case, is a theme made of twelve notes.  This is the twelve-tone system which was invented by Berg’s teacher Arnold Schoenberg.  By giving each character a theme, he is using the idea of a leitmotif, which was used a lot by Wagner. This can be heard in Scene I of Act II, when Lulu sings a dramatic solo telling Dr. Schön she has never pretended to be anything other than what she is. The first twelve syllables (If men have killed themselves for my sake...) that she sings are set to her leitmotif.