For Christ's sake, the word is equivalence, not equivalency. If one is going to act like such a smug know-it-all, then at least get some basic stuff right. Travis Johnson, Seattle.
Let me explain something, Travis. To be a smug know-it-all actually requires work. For a start you’ve got to be right at least most of the time, meaning you’ve really got to sweat the small stuff. So if you’re going to pick nits around grammar or language, then at least bother with basic research, rather than what you remember from elementary class. Equivalence and equivalency are variants of the same noun, as described by no less an authority than the Oxford Dictionary and, as such, are in flux in linguistic terms.