C minor: by far the most iconic Beethoven key. It's the key of the Fifth Symphony, the Third Piano Concerto, the Coriolan Overture, the Pathetique, etc., etc. - so much, in fact, that 'C minor mood' became a semi-official term in Beethoven literature. It's temptingly easy for to connect the brooding scowl on Beethoven's portraits and busts to the dramatic, stormy, high-intensity music he wrote in this key: works full of deep pathos and possessed by a relentless, sometimes demonic drive.
It's also temptingly easy to compare the C minor sonata, Op. 10 No. 1, with its younger sister, the Pathetique, Op. 13. The two are but a year or two apart and have a similar structure: a lyrical slow movement in A-flat major surrounded by an energetic first movement and finale. Perhaps, though, dubbing Op. 10 No. 1 'the small Pathetique' isn't that helpful. The earlier sonata may not possess the same catchiness or plumb the depths of human emotion to the same degree as Op. 13, but surely it has more than enough individuality to be loved for what it is, and not just as a precursor to a later work.
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