When in a recent post I used the word insure in a context that had nothing to do with underwriting, more than one reader wrote to chide me for not using the word ensure.
I’ll confess. The rule that insure must be used only in the context of indemnifying against loss is one that has never penetrated to my subconscious. I’ve read the rule. I’ve even written about the rule, but I can’t seem to rid myself of the idea that ensure is British spelling for insure.
The verb ensure entered the language in the Middle Ages. The earliest OED citation is 1385. The earliest documentation for insure is 1440. Until the 17th century, the forms ensure and insure were used interchangeably with a variety of meanings, including that of insuring a person’s life or property against loss.
According to OED citations, ensure was still being used in the 18th century with the meaning of buying insurance: “The price of ensuring the Life of a Man of 20 (1693).” And in the late 19th century, insure was still being used in contexts in which the modern rule requires ensure: “An ardour which could hardly fail to insure success (1862).”
The Penguin Writer’s Manual notes that the verb “generally used in the active form to mean “make (something) certain” is ensure,” but also notes that ensure is often spelled insure in American English.
For modern writers of American English, however, both The Chicago Manual of Style and The AP Stylebook urge the distinction:
Ensure is the general term meaning to make sure something will (or won’t) happen. In best usage, insure is reserved for underwriting financial risk. –CMOS
Use ensure to mean guarantee. Use insure for references to insurance. –AP
Use ensure when you mean guarantee. Reserve insure for talking about a financial arrangement meant to secure the payment of a sum of money in the event of loss or damage:
Worn or damaged parts are replaced to ensure the safety.
They go all the way to ensure the comfort and satisfaction of their passengers.
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Your renters’ policy will state exactly what you’re insured against.