word choice - Can someone explain when to use "relevance" and when "relevancy"?

Okay, so today I first came across the word relevancy in written. I grasped its meaning without problem, checking in a dictionary that my hunch was right.

So it would appear that relevance and relevancy both mean the same. They only differ in one letter and both are nouns (i.e. there is no distinction like adjective vs. adverb here).

All that said: what are the actual differences, however subtle, in the meaning of the two words? E.g. in what situations would I use one but couldn't use the other? Is one or the other used only in particular dialects of English (e.g. American vs. British vs. Australian)?

Are there even any differences? My rationalization for encountering both words would be that there must be a subtle difference as explanation why both words exist concurrently/

Answer

Relevance is the more common form, according to grammarist.com:

Relevance vs. relevancy:

• There is no difference between relevance and relevancy. Though the latter is the older form, relevance is now preferred in all varieties of English. In this century, relevance is about ten times as common as relevancy in U.S. popular usage, and the gap is even wider in British, Australian, and Canadian sources.

Ngram revevance vs relevancy

Etymology:

• relevance (n.) 1733; see relevant + -ance. Related: Relevancy (1560s).

-ancy (suffix):

• word-forming element denoting quality or state, from Latin -antia, forming abstract nouns on past participle adjectives in -antem, appearing in English mostly in words borrowed directly from Latin (those passing through French usually have -ance or -ence; see -ance).