In Washington, legislators are trying to “assure” their constituents that they are working to “ensure” that any new health-care bill will “insure” them.

All three of these transitive verbs mean the same thing: To make certain of something. (Surely you knew that.)

But there are subtle differences as well, which have evolved over the years.

Let’s start with the (relatively) easy one: “Assure.” It’s a transitive verb, to be sure, but its object should be personal—“I assure you” about something. You shouldn’t “assure” an inanimate object of anything. Yet many times “assure” is used when “ensure” is meant, as in “a new health insurance bill is supposed to assure that all people are covered.” Though Garner’s Modern American Usage says the substitution of “assure” when “ensure” is meant is “ubiquitous but …” (the verbatim Language-Change Index rating, meaning no one will be able to stop it, wrong though it may appear), the substitution does not appear frequently at all nowadays.

The differences between “insure” and “ensure” are also open to debate. “Insure,” some people say, must be used only where someone makes a legal wager that something will happen or not happen. People pay premiums for “life insurance” policies to ease the financial damage to their heirs; some financial companies have created complicated financial instruments that “insure” them against loss (though that bet did not pay off for many).

“Ensure,” some people say, should be used when the “assurance” is neither personal nor financial: “I will ensure that the next health-care bill will cover flu shots.”

The British did and do use “assure” in financial contexts where we Yankees would use “insure,” though Fowler’s Dictionary of Modern English Usage (second edition) pooh-poohed the usage. (Fowler did, however, distinguish between “life assurance,” which was “assured” of paying off eventually, and “term insurance,” which was a bet that someone would not die within a specified period.)

For many years “ensure” was viewed as a Britishism, possibly because of the “en” prefix, which sounds so, um, British. Until 1999, in fact, The New York Times mandated “insure” in both financial and nonfinancial contexts, though one of its main style gurus, Theodore M. Bernstein, was sure that there was no difference between the two words.

Nowadays, most usage authorities “assure” us that “insure” for “ensure” is perfectly fine: Garner’s supports “insure” only in financial contexts such as “life insurance,” but the Language-Change Index acknowledges the ubiquity of “insure” for “ensure.” Using “ensure” in financial contexts to mean “insure,” though, is just plain wrong.

For sure, while “assurance” and “insurance” are perfectly acceptable noun forms, “ensurance” simply doesn’t exist. But rest “assured”: Writers, for the most part, have “ensured” against its appearance.

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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.