Spellcheck probably won’t help you choose correctly among assure, ensure, and insure. In fact, because of some overlap in definitions, you might have some problems yourself deciding which word is the right one, especially if you’re working with historical texts.
According to Etymonline, ensure and insure both probably extend from the same Anglo-French root, which in turn may have been influenced by or been an alteration of an earlier word that developed into assure. So the meanings of these three words have long overlapped — and likely given pause to writers and editors for several centuries. But over time, they have more or less found their own identities.
Ensure vs. insure
The differentiation of ensure and insure is widely — though not universally — recognized among writers and editors:
Their differentiation occurred slowly and extended well into the twentieth century. The U.S. Constitution, for example, begins
We the People of the United States, in Order to form a more perfect Union, establish Justice, insure domestic Tranquility, …
By 1926, Fowler was offering the same guidance on differentiation that you find today — “In general usage insure is now limited to the financial sense, in which the form ensure is wholly obsolete” — but you can find well-respected, widely circulated, and closely edited publications using insure in a nonfinancial sense through the middle of the century:
To insure greater harmony and constant exchange of ideas, Lord Louis insisted on having the highest ranking U.S. and British officers live and dine with him. (“On the Plains of Delhi,” TIME, 29 Nov 1943)
Somewhere in the two volumes of the State’s history is mention of a custom — it has since died out — that wives of citizens were once in the habit of visiting the Castle with their newborn sons and lowering them gently into the Ross bowl, believing that by so doing they would insure an election to citizenship upon their offspring when they achieved riper years. (“Are We Worthy to Welcome Them Home?” The Saturday Evening Post, 17 March 1945)
But Fowler is certainly on target about where the overlap occurs: Insure sometimes appears in nonfinanical contexts; ensure used in financial contexts is a rare error that is perhaps a positive outcome of the ubiquitous nature of insurance commercials.
Assure most often means “give confidence to,” and in general should be used in that way”
I assure you that this example sentence was made up at the last minute.
But there is quite some overlap between assure and ensure, and even insure, especially in British English. Monty Python fans will recall that the short film at the beginning of The Meaning of Life featured an accountancy firm–cum–pirate horde called Crimson Permanent Assurance. And one can still buy insurance from the Mutual Assurance Society of Virginia — which, I should point out, was founded way back in 1794, when a young nation was still striving to insure domestic tranquility.
Also, the set phrase “mutually assured destruction” (sometimes “mutual assured destruction”), which dates back to World War II, uses a definition of assured that more closely aligns with what we expect of ensured today.
Language change and cross-cultural differences sometimes mean that best word choice is not always unambiguous. English is funny that way. But in modern writing, you’ll tread on safe ground if you insure your home to ensure you’re compliant with state law and to assure your family that they’re financially protected from disaster.