Adjectives ending -ant seem to have a 50/50 chance of ending -ancy or -ance in noun form. Same goes for -ent adjectives into -ency or -ence nouns. Is there a rule that determines which?
Random examples:
- bouyant becomes bouyancy
- exuberant becomes exuberance
- potent becomes potency
- impotent becomes impotence
- brilliant becomes brilliance
- ascendant becomes ascendancy
I have the feeling that the noun takes the -ancy form when it denotes a quality which has degrees, whereas the -ance form is more common when the quality is binary (e.g., drugs having varying levels of potency, but men are impotent or they are not). However, there are certainly some counterexamples (e.g., importance).
The reason I am asking this peculiar question: in mathematical jargon, the words nilpotency and nilpotence are both somewhat common. I want to know if it's reasonable to use the two words in slightly different ways. I haven't thought of an ordinary word for which both noun forms are used (possibly with different meanings), so probably I should pick one and stick with it.
This question is related but about -ance vs -ence: Is there a rule for “‑ance” vs. “‑ence”?
