In the excellent dvd set, “Haydn, The String Quartets”, the first violinist of the Lindsays (string quartet) comments that f-minor was Haydn’s “personal” key; this in reference to the Quartet op. 20, Nr. 5. He goes on to suggest that c-minor and g-minor were Beethoven’s and Mozart’s “personal” keys, respectively.
He’s right. But a “personal” key is by no means the most ubiquitous key in a composer’s output; if it were, just about all classical composers would have personal keys of C or D major. The key word is indeed “personal”…in Haydn’s case, consider the piano variations in f-minor and the symphony “La Passione” in addition to the quartet. In Mozart’s case, the 40th symphony, g-minor quintet and Papageno’s suicide music. In Beethoven, you can start with the 5th symphony and go from there.
More interesting is the pschological mood these keys denote, not just their technical character. For Haydn, you might say “passion”, for Mozart “despair”, and for Beethoven, “struggle”.
I suggest other personal keys: for Chopin, B-major, (and, by the way, Chopin insisted that B major was the easiset and most natural scale on the piano); for Liszt, F#-major, for Tchaikovsky, b-minor, for Janacek, D flat-major. For starters. You will notice that all of these keys are relatively exceptional keys compared to the general ubiquity of keys in the repertory. For those who need persuading about the above designations, consider the following:
Chopin: 3rd sonata, numerous nocturnes, waltzes, and mazurkas, as well as sthe still heart of such pieces as the Fantasy and Poloniase-Fantasy.
Lizst: The “Lucifer” music in the “Dante Sonata”; the “Mephistopheles” music in the b-minor sonata, the Mephisto waltz, etc.
Tchaikovsky: Swan Lake, “Manfred” symphony, and especially the 6th symphony.
Janacek: Sinfonietta and the great epiphany from The Cunning Little Vixen.
But one can make their own list. Mahler doesn’t have a personal key, possibly because all of his music is so personal, so egregiously autobiographical.
In assigning “personal” keys, one should keep in mind that it is a sort of parlor game, and different assignments and disputations are inevitable and welcome; neither is choice of key the most important element even in those works for which a strong identification is present. But it can guide us to certain discoveries, and provide a shorthand for describing the aesthetic predispositions of certain composers. Cross comparison is interesting as well. I suspect most experienced musicians would regard b-minor as moody, even apart from Tchaikovsky…that F#-major is ecstatic, or Promethean…by the way let’s absolutely put Scriabin in the F#-major camp, he belongs with Liszt. It seems like every other piece by Scriabin has the totemic 6 sharps.
A final comment: it is no coincidence that for the classical masters, Haydn, Mozart and Beethoven, their characteristic keys are minor keys. In the Classical era, minor keys were already exceptional, and therefore potentially “personal”.