Hey, folks. This is another one of those discussions where there’s a mostly right answer to the question, but not a wholly definitive one. (Not to my mind, anyway. I know what I prefer, but that don’t make it right, as they say.) And that makes it especially fitting for Homophone Hell, because . . . there’s no way out. ::cackles::
The title should have clued you in pretty well. Is the phrase “free rein” or “free reign?” The answer is “yes.” (I sighed heavily as I typed that, just so you know.)
“Free rein” comes from the equestrian realm. It’s a rein held loosely, giving the horse relative freedom (until he’s reined in). The phrase can be applied to other entities as well, not just horses. A manager may be given free rein to make her own decisions, until her superior thinks she’s gone too far — at which point he reins her in.
“Free reign” is the alternate wording, and there’s a great discussion of it here at Daily Writing Tips. It discusses the process of conflation between “rein” and “reign” clearly, and give supporting evidence in the form of search-engine hits. “To reign” means “to wield the power of a king” — I can see how some folks made the jump from giving a horse (or a person, or a corporation, or a whatever) its freedom to do what it wants, but . . . anyway. I won’t rehash the entire post. I will, however, offer the ending quote of the original post (there’s an addendum), with which I fully agree:
“I shall continue to write ‘free rein,’ but free reign is here to stay.”