The truck on the highway carrying dangerous chemicals usually carries a notice that its contents are “inflammable.” If the truck were very short, it could just as easily carry a sign that the contents were “flammable.” The words mean the same thing: Please don’t throw a lit cigarette into that direction, or boom!

The financial executive who has combined accounts to keep fees low for clients hasn’t just “mingled” the funds; he has “commingled” them. Again, they mean the same thing: The money is all mixed together.

English has many of these combinations, where a prefix doesn’t change the meaning of a word, only modifies it slightly (think “loosed” and “unloosed” and the not-quite-a-word “irregardless”). Sometimes there’s a nuance to be gained, but other times it results only in confusion.

“Inflammable” is probably the best (confusing) example. The “in-” prefix has two main—and nearly opposite—functions. One is to make a word negative: Add “in” to “direct” and the result, “indirect,” means “not direct.” The other is to give a word an intensity: Add “in-” to “flame,” and the result, “inflame,” means something more than a mere flame.

But most people think of the “in-” prefix as a negative, especially when attached to a common word, like “disputable,” “consequential,”—or “flammable”—so it takes a second to remember that “inflammable” doesn’t mean “not flammable.” And though you might think “inflammable” was a back formation of “flammable,” it’s the other way around. As Garner’s Modern American Usage notes, “Traditionally the forms were inflammable and noninflammable; today they are flammable and nonflammable.” For many people, though, it’s too hard to parse what looks to be a double prefix, “non” and “in” with “flammable, come up with “not not flammable,” and realize that means this thing burns. Hence “flammable” was born. (A similar dislogic has been discussed here with “disembark.” And yes, we made up the word “dislogic.”)

But not everyone has quenched the fire. News reports often talk of things that are fireproof, but use “inflammable.” And children’s sleepwear has to meet “flammability” standards (pdf), which call for the pj’s to not be “flammable” at all—they must be “flame resistant and self-extinguish” if exposed to fire. So why not make them “nonflammability” standards, so people wont think the standards are how easily those clothes burn? The “non-” prefix is almost always a negative.

The “co-” prefix is another brain-twister. Usually it means something like “shared,” as in “co-author,” or in “coeducational,” shortened in American English to “co-ed” or “coed,” meaning a female who’s attending a school where the sexes are mixed. The definition of “commingle” shows the dislogic of “commingle,” since it means “to mingle together,” even though “mingled” things already are together.

Though “commingle” goes back to Shakespeare’s time, today it is used almost exclusively in financial contexts. And the “mm” spelling has trumped the older spelling, “comingle.”

As for “loosed” and unloosed,” the dislogic is similar to that of “inflammable,” in that “un-” usually means “not,” causing momentary confusion unless the context is clear. “Unloose,” though, goes back to the 1300s; “loose” is about a hundred years older, so that one’s here to stay. And please, don’t confuse “loose” with “lose,” or we’ll set the dogs on you.

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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.