Ensure and insure are pronounced the same way and grew from the same root, so it's natural to wonder whether they're two spellings of one word. They aren't quite, and the answer depends partly on where you are.

Ensure means to make sure. You ensure the door is locked, ensure everyone has a copy, ensure the data is backed up. It's about making something certain, with no money involved.

Insure is the insurance word. You insure a car against theft, insure a house for a set sum, insure yourself against illness. An insurer is the company that sells the policy. Whenever premiums, claims or policies are in the picture, insure is the only choice.

The overlap comes from North American usage, where insure can also mean make sure. Learner's dictionaries record that sense as especially North American, which is why you'll see "insure that everyone arrives safely" in American writing without anyone blinking. In British English, that use reads as a mistake.

The simplest policy is portable: use ensure for "make sure" everywhere, and insure only for insurance. That is correct on both sides of the Atlantic.

Memory trick. Insure and insurance begin with the same six letters, so keep insure for policies and premiums. Everything that just needs making certain gets ensure: it has an e for every other case.