We made a mistake in a headline on Page 1A in The Sun on Nov. 16, "Senate moves to tighten reigns on wartime policy." The copy editor who wrote the headline — one who knows better — wrote reigns instead of reins. Sometimes, particularly when the work is done hurriedly, the wrong synapse fires.
But the error is one of a class, confusing two homonyms, that is particularly difficult for writers and editors. The wrong word spelled correctly will not be caught by a spell-checking function, and electronic grammar-checking functions are notoriously unreliable.
Reign and rein belong to the subclass of homonyms called homophones; they sound alike but have different meanings and spellings.
The other class, homographs, includes words that are spelled alike but have different meanings, and sometimes pronunciations. Lead, the metal, and lead, the verb, belong in this class. But lead, the metal, and led, the past tense of the verb lead, are homophones.
For a useful electronic list of homonyms, have a look at Alan Cooper's Homonyms.
The other problem with reign and rein is that the usage that gets confused most commonly is a buried metaphor. We no longer ride horses much, but we still take the reins when we assume control or give free rein when we surrender control to someone else. The English language is full of stock expressions that started out as metaphors and have since worn smooth with use.
Those buried metaphors frequently trip up writers who convert an expression they have imperfectly heard into written language. When I taught freshman English at Syracuse 30 years ago, my jaded sophisticates would observe that it is a doggie-dog world out there. Not an attractive picture, but certainly less disturbing than the dog-eat-dog world envisioned by Hobbes and others.