Ben Zimmer in the Oxford University Press blog:
In our last installment, I noted that the increasingly common spelling of minuscule as miniscule is not just your average typographical error: it makes sense in a new way, since the respelling brings the word into line with miniature, minimum, and a whole host of tiny terms using the mini- prefix. It might not be correct from an etymological standpoint, since the original word is historically related to minus instead of mini-, but most users of English don’t walk around with accurate, in-depth etymologies in their heads. (Sorry, Anatoly!) Rather, we’re constantly remaking the language by using the tools at our disposal, very often by comparing words and phrases to other ones we already know. If something in the lexicon seems a bit murky, we may try to make it clearer by bringing it into line with our familiar vocabulary. This is especially true with idioms, those quirky expressions that linger in the language despite not making much sense on a word-by-word basis.
Take the idiom free rein, meaning ‘freedom of action or expression.’ Our blog editor Becca Ford recently passed along the usage tip from Garner’s Modern American Usage, which advises that this is the correct form, not free reign. “The allusion is to horses, not to kings or queens,” Garner writes, “but some writers have apparently forgotten the allusion.”