The turning point in modern classical music was Claude Debussy's Prelude a l'apres-midi d'une faune. After this piece, music was never the same. The main factor involved was the entirely new concept of thinking of harmony as non-structural, which meant that any set of sounds could lead to any other, depending on the intention (and skill) of the composer. Before, a chord was first and foremost considered as part of a key, and either prolonged a passage or led to some sort of cadence. Chords were labeled with roman numerals and elaborate names, which by the very act of doing so, implied how they were to be used. Listen to this piece and compare it to anything by Mozart, for example, and you'll hear the difference immediately. Tomorrow: more on Debussy and a discussion of Les Ballets Russes.