People sometimes confuse reign, rein, and rain, and sometimes it's not obvious which word to use in a metaphorical sense. Here's a quick rundown.
Do you want to hold the reins, reign over the land, or see a little rain fall?
Reins are the straps you use to control a horse, and the word rein comes to English from a Latin word that means "to hold back." Remember that rein in is the correct spelling by thinking of the troublesome person you need to rein in as a troublesome horse that you need to get under control.
Rein is also the spelling you want in the phrase free rein. When you have a horse you trust, you give it free rein—the freedom to make its own way. Think of your friends and coworkers as horses again when you're trying to remember the spelling. When you trust them enough to do things on their own, you're giving them free rein.
In contrast, when you watch or control people carefully, you keep them under a tight rein—again, just like a horse.
Reigning is something a king or queen does. The period of time during which a monarch rules a country is also referred to as his or her reign.
Reign of Terror
The phrase reign of terror was first used to refer to the particularly violent time at the beginning of the French Revolution.
Reign of Terror is capitalized when referring to these events for the same reason we capitalize Boston Tea Party and Industrial Revolution: It's the name of a major historical event. (1) Keep it lowercase when it refers to a generic event: Our HOA president's reign of terror continues.
Rain was first used to describe the wet stuff that falls from the sky, and later people added figurative meanings such as describing things that appear in abundance: It's raining men! Hallelujah!
He Rained Terror
On Twitter, @SAlexM23 asked a question about He reigned/rained terror:
It's a tricky question since it makes us think of the set phrase reign of terror, but it isn't identical. Although Nelson Mandela famously said, "Let freedom reign,' giving us the image of freedom ruling or reigning over the land, when reign is used as a verb, it doesn't usually fit with an object such as terror. He reigned freedom doesn't sound right. You reign something, such as a country. You don't reign something on something else. That kind of construction fits more with rain: The clouds rained water on the earth. It's not unheard of for rain to be used in a figurative sense. For example, in Henry IV, Part 1, Shakespeare wrote It rained down fortune showering on your head. (2)
A Google Ngram search is also somewhat helpful. Although reigned terror was more common in the past, rained terror is more common today.
You could write He reigned with terror or His reign was filled with terror, but I'd stick with rain for a sentence such as He rained terror on the land.
1. "Historical events and programs." Chicago Manual of Style, 16th edition. Section 8.74. http://www.chicagomanualofstyle.org/16/ch08/ch08_sec074.html (accessed May 6, 2014).
2. "rain." Oxford English Dictionary, online edition. http://www.oed.com/view/Entry/157580 (accessed May 6, 2014).
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