While both words derive from the Latin word securus, meaning to make safe, sure, or certain and were once used interchangeably, they are now used differently by most writers and publications. Our advice is to stay in that mainstream.

When you ensure that you will have a report ready by Friday, you are guaranteeing that the report will be finished by Friday. You are pledging to do everything needed in order to have it ready at the designated time; you are promising a certain outcome.

But you insure against loss. You purchase insurance so that you are insured. “Insure” carries with it a financial context.

And although it falls outside a pairing of easily confused words, we’ll toss in “assure” as a little lagniappe (a word used mostly in the US South to mean “something extra, a small bonus”). Assure, too, derives from securus but means “to remove doubt about, to state with confidence, to make something certain to happen, to cause to feel sure.” So it is that your promotion may be assured, you may assure a colleague that you will attend a conference, and you had better assure your mother that she makes the best meatloaf in the world.

There’s a generally accepted rule for this one that isn’t too hard to master once you recall the difference between restrictive and nonrestrictive clauses. “That” is used in restrictive clauses and “which” is used in nonrestrictive clauses.

A restrictive clause is a part of the main sentence that contains information necessary to the meaning of the sentence. It “restricts” or limits the noun that comes just before it, the noun it modifies. (Both of the preceding sentences contain restrictive clauses.)

Here is another example:

• The regulatory commission that will decide the case has a history of focusing on Patient Reported Outcomes (PROs).

Here, the restrictive clause limits the meaning of “regulatory commission” to the one tasked with deciding the case. It implies that more than one regulatory commission exists and so provides essential information about the one mentioned.

But a nonrestrictive clause provides nonessential information, information that can be omitted without changing the meaning of the sentence.

• The regulatory commission, which will decide the case, has a history of focusing on Patient Reported Outcomes (PROs).

Here, the clause is nonrestrictive – it provides extra rather than essential information. Its implication is that only one regulatory commission exists or that the audience already knows which one is under discussion. The “which” clause acts almost as an aside.

Note that nonrestrictive clauses require commas.

When you need to restrict meaning, use “that.” If you don’t, use “which.”

The increased understanding that results for your audience strengthens your effectiveness as a speaker and writer. Ensure it.