San Francisco commuters were relieved recently when a commuter rail strike was averted. But for some time, stories about the negotiations said that rail workers had vowed “to strike at midnight Sunday.”

To some readers, that meant that they would not be able to take the train to church on Sunday; to others, it meant that Monday’s work commute could be problematic.

“Midnight Sunday” could mean between Saturday night and Sunday morning, or between Sunday night and Monday morning, depending on how one thinks of “midnight.” If it’s already Sunday, no one’s going to be confused; but if it’s Saturday or before, readers might not know which “midnight” to worry about. (In this case, the strike was planned for between Sunday and Monday.) While some dictionaries and usage guides say that “midnight” looks forward to morning, since the clock changes from p.m. to a.m., others, including Webster’s New World College Dictionary, say that midnight means “twelve o’clock at night; the middle of the night,” looking backward instead of forward.

Many news publications get around the issue by saying “12:01 a.m. Monday,” which eliminates the confusion but introduces a slight inaccuracy. Strikes generally don’t begin at 12:01 a.m., they begin at midnight.

Another way to avoid the problem is to write that the strike is threatened for “overnight Sunday” or “first thing Sunday.” While this is also slightly inaccurate, in that it doesn’t give a specific time, it would clarify things for readers wondering whether there will be a train to catch.

“Midnight” isn’t the only two-faced word, of course. Quick: What’s a “biweekly” publication? Twice a week, or twice a month? What about a “biannual” art exhibit? A “biennial” one?

“Biweekly” is supposed to mean once every two weeks, or twice a month (in most months). But the prefix “bi-” means “two”—if you have “bifocals” you have two glasses in one frame—so you can see how someone might believe that “biweekly” means twice a week. And, as if that weren’t confusing enough, “biannual” really does mean twice a year; the word for every other year is “biennial.” (“Biennial” can also mean “lasting for two years.” Go figure.)

As with “midnight,” when there is any possibility of ambiguity, it’s better to avoid those words entirely, writing instead that “the magazine publishes every other week” or “the art exhibit is held twice a year” (or every other year).

The prefix “semi-,” which means “half,” doesn’t have as many identity problems as “bi-,” but some people—and dictionaries—think a “semiweekly” publishes every other week, not twice a week. And since “semiannual” and “biannual” mean the same thing—twice a year—who can blame them?

And let’s not even get into why “flammable” and “inflammable” share the exact same meaning. It’s too easy to get burned.

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Merrill Perlman managed copy desks across the newsroom at The New York Times, where she worked for 25 years. Follow her on Twitter at @meperl.