See the leather straps crossing the horse’s neck horizontally?. Those are reins, held by the rider and used to direct the horse left or right.
Raise your hand if you have held a set of horse reins in the last year. Yeah, about what I thought — not many of you have.
If you hold them loosely, letting the horse (or team of horses pulling a carriage, etc.) go as fast or slow as they want and where they want, you have given it “free rein.”
Not “free reign.”
Note the woman at right. She is Queen Elizabeth II, of whom it is sometimes said (as of other recent British monarchs) that “she reigns but does not rule.” In other words, she is the head of state, but she cannot make law nor order “Off with his head!”
No show of hands about the queen, sorry. But the difference is clear enough.
So when you want to restrict the spread or movement of something, you “rein (it) in.” Or someone gives you a power to do what you like — gives you “free rein.” But it has nothing to do with being a monarch.
Yet I have found that egregious “eggcorn” in two scholarly books within the last two weeks, one from Johns Hopkins University Press and the other from Thames & Hudson, which “has always prided itself on the very high standards of the books it produces.”
The real lesson here is that writers should avoid those metaphors that are dead to them — or they make silly errors. And careless editors let them go through.
For instance, I have shot antique-style flintlock guns, so I know what a “flash in the pan” looks, sounds, and smells like. It is a partial ignition of the priming powder that fails to set off the main powder charge and fire the gun. In other words, a promising start that goes nowhere. It has absolutely nothing to do with gold-panning, despite what some people think.
If you have visited a steam-powered railway, like the Cumbres & Toltect, you have seen a coal-burning locomotive “get up a head of steam”—build sufficient steam pressure—before it begins to move. But should you use such an expression if it leaves some readers puzzled?
If your audience isn’t horsey, don’t give them free rein.