This applet will determine the prelude or fugue number in Bach's Well-Tempered Clavier or Chopin's Preludes or Hindemith's Ludus Tonalis, given the key. It will also take the prelude number and return the key.
In the case of the Hindemith, there is no separation of major and minor mode (the fugues are not in traditional harmony, and all end with a major triad), so selecting major and (parallel) minor will both return the same number. When translating the Hindemith fugue number back into the tonic pitch, the applet will always return the major form of the key. The prelude/fugue number should not exceed 24 for Bach and Chopin, and 12 for Hindemith. All three buttons are toggle buttons. I don't recommend selecting more than one at a time, but it doesn't mess anything up if you do.
There is a graduate school "urban legend" that music faculty tend to ask graduate oral exam questions about the key scheme Bach and Chopin used to organize their sets of preludes (and fugues). In general, Bach's Preludes and Fugues are in parallel major and minor pairs, and proceed chromatically from C major through B minor. Chopin's preludes are in relative major and minor pairs and proceed through the circle of fifths (C, G, D, A, etc.). For mathematical formulae to use to do the conversions demonstrated in this applet, or to see a chart of the key scheme for all three of these works, see my page on Traditional Key Scheme Patterns for Works in All 24 Major and Minor Keys.
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