Brad Stolzer wonders about ensure and insure:
Am I the only one who struggles with these?
Not at all. And while we’re at ensure/insure, let’s throw in assure.
All three words are close in origin and meaning.
Both assure and ensure came into English in the late 1300s, assure from Old French asseurer, “to reassure, calm, protect, to render sure,” and ensure from Anglo-French enseurer, “to make sure.” The word insure appeared about 1440 as a variant of ensure. It took on the sense of “to make safe against loss by payment of premiums” in 1635. Before that, assure had that meaning.
In modern usage, insure has won out as the word that has to do with compensation for financial loss. The violinist insured his hands with Lloyd’s. This use of insure applies on both sides of the Atlantic.
The confusion that arises with insure vs ensure stems mainly from another definition: “to make certain that (something) will occur.” For example: We wish to ensure the safety of our passengers. Some speakers of American English would use the spelling “insure” in this context, but others might, like speakers of British English, write “ensure.”
The AP Stylebook offers these guidelines:
Use ensure to mean guarantee: Steps were taken to ensure accuracy.
Use insure for references to insurance: The policy insures his life.
According to the Chicago Manual of Style, “we ensure events and insure things. But we assure people that their concerns are being addressed.”
The Associated Press Stylebook
The Chicago Manual of Style
The Online Etymology Dictionary
The Oxford English Dictionary
The Penguin Writer’s Manual