Question: “Dear Grammar Guy: I got burned the other day trying to use the expression ‘free reign’ – or, rather, ‘free rein,’ as one of my friends snidely pointed out. Which is the correct version? ‘Free reign’ makes sense to me.”

Answer: Alas, dear reader, I’m sorry to say your friend has the correct answer on this one. I see where your confusion may be coming from, though.

There are two expressions to consider here: “to give free rein” and “to rein in.” Both come from horseback riding, where reins are used to control and guide the horse (along with a riding crop and possibly a carrot and stick, depending upon the metaphor at hand).

In our expressions above, we understand the reins to be figurative: they are a metaphor for power and control. But does “reign” not mean “dominating power or influence?” Can it not be used as a verb to mean “to have control or rule?” Why aren’t they interchangeable?

The simple answer is: Because that’s not where the expression came from.

A longer and perhaps more elucidative answer is that “reign” may only be used as an intransitive verb – that is to say, you don’t reign something, you simply reign. “Rein,” on the other hand, may be used transitively or intransitively, which allows us to have the direct object we need for the expressions to make sense (ex. “We reined in our out-of-control budget.”).

The takeaway from this is that you don’t need to remember all the transitive/intransitive stuff, you only need to remember that, when it comes to the expressions “free rein” and “to rein in,” it’s always “rein.”