The expression free rein stemmed from horseback riding.
The rider holds the reins, leather straps connected to a metal bit in the horse’s mouth, and handles the horse by pulling on the straps differently. When you give a horse free rein, you’ll hold the reins loosely so the animal can wander where it wants. In contrast, if you’re keeping a tight rein on something, you manage its every move.
Free rein is an efficacious way to say “freedom” in a sense where that freedom has been issued or permitted by a person or establishment; you’ll mostly see this phrase after some form of the verb give, like this first example below:
Pollution measures were relaxed, and development projects were given free rein to operate in woodlands and other natural areas, contributing to deforestation. (Slate)
Now he had free rein to explore the whole house. (The Marvels)
It’s easy to understand why somebody would confuse the accurate expression, free rein, with free reign. Reign and rein are homophones, and both words deal with forms of control. Reign means “to rule,” as a king or queen does. As a noun, it refers to the period when a monarch rules or royal authority more commonly. By definition, a monarch would already have free reign because they are the ruler, the supreme authority in their kingdom. So free reign is repetitious.
Free reign is a typical mistake, however, and even editors who should know better will sometimes not catch it:
Instead, the teachers are offered free reign to create their curriculum based on things they want to learn more about. (Salon)
She’s been given free reign to let her imagination run wild when she’s hired. (Los Angeles Times)
If free reign is incorrect, then so is reign in, another frequently misused phrase. To clear any confusion, remember that the correct expressions, free rein and rein in, have origins in horseback riding.
Related: Check out our “common expressions” section for more articles like this one!