Be honest now: do you know the difference between i.e. and e.g.? A lot of people think the two are virtually the same, but if they were, we’d only need one of them. So let’s break it down, once and for all.

Writers use i.e. to restate the subject at hand: A good Samaritan (i.e., my neighbor Blake Smith) drove my cat to the vet. In that sentence, i.e. tells the reader exactly who the “good Samaritan” was. One should use i.e. to identify, amplify, clarify, specify, or any combination thereof. Its purpose is to ensure that the reader knows beyond a doubt what or whom the writer is talking about.

The initialism i.e. is from the Latin id est, which means “that is.” In American English the i and the e are each followed by a period, and i.e. should be followed by a comma. Many authorities, including the redoubtable Chicago Manual of Style, discourage the use of i.e. in formal writing, advising that is instead. If for any reason a writer deems it necessary to use i.e., it should appear in parentheses: Winston Churchill spoke often of his “black dog” (i.e., his gloomy periods).

Writers use e.g. to give specific examples of the subject at hand. It is short for exempli gratia, a Latin phrase meaning “for example.” The e and the g are each followed by a period, and e.g., like i.e., should be followed by a comma. In formal writing it is advisable to write for example or for instance instead of e.g. But if a writer insists on it, e.g. and the example(s) that follow it should be placed in parentheses: High-fiber foods (e.g., lentils and broccoli) are good for you.

Sometimes the right choice requires careful thought, as in this case: Certain members of my family (i.e., Mom and Uncle Jake) are vegetarians. In that sentence, the i.e. tells us that Mom and Uncle Jake are the only family members who don’t eat meat. But what if we replace i.e. with e.g.: Certain members of my family (e.g., Mom and Uncle Jake) are vegetarians. Now the sentence means that there are other vegetarians in the family besides Mom and Uncle Jake.

That is no small difference, and it highlights the dissimilarity of i.e. and e.g. Confusing one for the other can result in misunderstandings at best and nonsense at worst. So remember to use i.e. when further identifying a subject, and use e.g. when giving specific examples of a subject. A handy memory aid:
e = “example,” i = “identify.”