I recently saw someone use the word "elegancy" for the first time in forever, and it set me wondering about (and wandering about) why the synonym "elegance" appears to be the preferred noun. I searched the web for some kind of standard treatment of the difference, and I could find none. A [structurally] similar pair of nouns, "lenience" vs. "leniency", seems to find the "-y" ending to be favored.
Are there standard phrases where "elegancy" would be the preferred usage (similar to how judicial applications seem to strongly favor "leniency")?