Patreon Patron, Mr. Franklin, asks:
“Dr. B – how about a post/lecture discussion about WHY certain composers or genres chose specific major or minor keys. To state the obvious, the intervals between the pitches in all major keys are the same whether the tonic is C or Ab. Why choose one over the other? Thanks.”
This is an excellent question, one I am asked rather frequently. I would take this opportunity, then, to go on the record with
Tuning Systems
Ron Franklin correctly observes that in the equal tempered tuning system that is standard today, the intervals in all the major (and minor) keys are exactly the same. However, despite its adoption here and there during the late eighteenth and early nineteenth centuries, equal temperament did not become the gold standard, default tuning system in Europe until the mid-nineteenth century. The “system” that existed to that time was something called “well temperament”, which in fact is not a single tuning but rather, an umbrella term for various tunings in which certain keys were rendered incrementally brighter and darker by enlarging or shrinking certain intervals
“There were many unequal [well-tempered] tuning systems around in the eighteenth and nineteenth centuries, each with its fierce partisans and enemies (tuning has always been a spur for fanaticism.)”
For example, J.S. Bach was very particular about his tunings. In 1776, the German music critic, theorist, and composer Friedrich Wilhelm Marpurg wrote:
“Mr. Kirnberger [Johann Philipp Kirnberger, a student of Bach’s] has more than once told me (as well as others) that the famous Joh. Seb. Bach confided to him the tuning of his clavier, and how that master expressly required of him that he tune all the thirds sharp.”
Bach’s son Carl Philip Emanuel (C.P.E.) recalled:
“The exact tuning of his instruments as well as the whole orchestra had his greatest attention. No one could tune his instrument s to please him. He did everything himself.”
During the seventeenth and eighteenth centuries, whole treatises were written by such music theorists and composers as Andreas Werckmeister, Johann Friedrich Mattheson, Friedrich Marpurg, Johann Adolf Scheibe and J. J. Quantz, describing what sorts of tunings and which key areas were most ideally suited to creating particular “
Continue reading the answer and discussion of the issues on Patreon
Just as the French artist Charles Le Brun (1619-1690) cataloged the “passions” (archetypal emotional states) and indicated just how those passions might be illustrated, so such seventeenth and eighteenth century music theorists and composers as Andreas Werckmeister, Johann Friedrich Mattheson, Friedrich Marpurg, Johann Adolf Scheibe, J. J. Quantz and Francesco Galeazzi described what sorts of tunings and which key areas were most ideally suited to creating particular “affective” (emotional) states. See Le Brun’s “Jealousy”, “Sadness”, “Fright”, and “Admiration” below:
Charles Le Brun, “Jealousy” 
Charles Le Brun, “Sadness” 
Charles Le Brun, “Fright” 
Charles Le Brun, “Admiration”