Reign means to rule as a monarch. A rein is a strap used to control a horse. The correct phrase is free rein — loosening the reins so the horse chooses its own pace. "Free reign" is a very common error, and understandable, since giving someone free rein does look like letting them rule.
The core difference
One is rule, the other is control by strap — and the horse metaphor owns the phrases.
- reign — to rule: a long reign, the reigning champion.
- rein — a control strap; to restrain: rein in spending, free rein, a tight rein.
The phrases and why they take rein
Free rein, rein in, a tight rein, keep a rein on — all from horsemanship, all spelled rein. Once you see that they belong to one metaphor, they stop being separate things to memorise.
The one that takes reign
Reigning champion — currently holding the title, as a monarch holds a throne. That is the exception, and there is also rain, a third homophone, which appears in rain check and take a rain check.
Frequently asked questions
Is it "free reign" or "free rein"?
Free rein — from loosening a horse's reins to let it choose its pace.
Which phrases use rein?
Free rein, rein in, a tight rein, keep a rein on — all from horsemanship.
When is reign correct?
For ruling — a monarch's reign, the reigning champion.