Debussy had been planning Preludes for two or three years before the first book was published in 1910. He seems to have been at work on the second book by the end of 1911 and the volume was published in 1913. Whereas Bach and Chopin used their Preludes as something of a technical exercise, composing one for each musical key, Debussy’s were free-form and full of his signature expressive indulgences. Each one has a different descriptive title that gives clues about what was going through the composer’s mind when he was writing them, from Shakespearean dances to underwater cathedrals. What Debussy did in the Preludes and earlier works with a similar inspiration was composing with sounds rather than notes. It is that, rather than their picturesque qualities, that changed history. Debussy did nothing less than liberate music from the domination by functional harmony which had prevailed for three centuries. Tonality becomes color.