What a thrill when my sixteen-year-old began her lesson yesterday with an astute question about Beethoven's Op. 14 No. 1 Sonata in E: "Why does he start off the Development section as if he is going to stay in E then go off into C major?"
By the end of the lesson we had explored all the sharps of E major in relation to tones in the C major scale. I have long realized that Mozart writing in the key of C uses accidentals that tie C to E major--D# is not infrequent as a first accidental in pieces in C. It is as if Mozart is using E as a covert key while Beethoven uses C as the covert key.
What fun to see a young person excited about Beethoven through personal experience rather than by some kind of theoretical approach.