A tangent to THIS thread about freedom of pets in houses. I saw both phrases, and was curious (and bored).
Free rein, as in “I took the carriage to the general store yesterday and gave the horse free rein, and she she delivered me…straight to Slippery Sally’s Saloon.”
I wish when I gave my car free rein it would take me to a saloon. Mine takes me to work. 
(That’s what I call it when I space out and drive without thinking about where I am going. Also known as auto pilot.)
I vote for “free reign” but so far it’s only me.
A google search for “free rein” gets 808,000 results. “Free reign” gets 570,000. So I’m in the minority, but not entirely alone.
I’ve always understood it as “free rein,” with the analogy of letting the horse go where it will and not guiding it with the reins…
You could tie the team’s reins loosly and just let the horses find the way home and you would wake up when they got there. The horses would find the way and so you have given them “free rein”.
I think of it as “free rein,” as in horses, as well.
This poll leaves out the option of “free rain”
If you want some, I’ve had plenty.
Yeah, free rain. That’s it kind of like Purple Rain, Purple Rain except it doesn’t cost any money. Free Bird! Rock on.
570,000 people on the Internet CAN be wrong.
Never thought about it before, but I’d say “free rein” for the dog as in “dog unleashed” and “free reign” for the cat who lords over us whether we like it or not.
Rein. Horses.
It is absolutely and without a doubt “free rein”.
The increasing use of “free reign” drives me nuts and I bitch about it regularly.
Dallas Jones, don’t you dare get that started. I will gnaw on your skull!!
That’s pretty much where I am with it, but I figured that before I got my knickers in a knot I’d see if it was a regionalism or something that I hadn’t come across before.
There are quite a few "reign"s in the original thread.
Free reign on a project which means I have total control over it.
Are we actually voting on reality now?
It is “free rein”, and if this didn’t have a poll attached I would move it to General Questions, since it has a factual answer.
indeed it does.
n.
Unlimited freedom to act or make decisions: gave me free rein to reorganize the department.
I guess my version would be paid reign.
I voted “free rein” but I can understand the argument for the other option.
Unlike the " . . . you have another think/thing coming" debate. Which very obviously has people confused as to the true intent and meaning of the phrase.
…I’ve never actually thought about it.
Have you tried cruise control?