Articles about keys tend to remain unread, even by musicians, who will pick out the odd point of interest, but regard tonal structure as something to be heard and not seen, as indeed it is. It's even true to say that most articles on keys are, in fact, boring: they explain, in infinitely patient conceptual detail, what is part and parcel of one's instinctive musical experience anyway, if one is musical at all—so why bother?

However, my subject is not a key, but a non-key—the most dramatic non-key I know in our entire diatonic literature. No technical knowledge is needed to appreciate what is a 181-year-old sensation. All you need to know is what one sharp means in a key signature. For the rest, it's worth getting a score of Cosi fan tutte out of the library if you don't possess one.