What’s the story, is it free rein or free reign? Something that has confounded me for a good while.

To me, both spellings and implications make sense. To give someone free rein is to loosen the metaphorical reins to the point where they can do as they please.

To give someone free reign is practically the same thing, though it sounds more regal, more majestic if you like, reflecting the meaning of the word spelled that way.

Apparently the former is the correct spelling and use of the phrase. See here from Merriam-Webster, whose US provenance doesn’t negate the conclusion for UK English I don’t think.

Somewhat tangentially, in fact totally tangentially, here’s a question whose answer you should file away for potential pub quiz benefit. What’s the only word in the English language which ends in mt? Answer – dreamt! Compound verbs featuring dream don’t count :-).