What to Know

I.e. is an abbreviation for the phrase id est, which means "that is." I.e. is used to restate something said previously in order to clarify its meaning. E.g. is short for exempli gratia, which means "for example." E.g. is used before an item or list of items that serve as examples for the previous statement.

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Latin may be a language of antiquity, but a healthy number of Latin abbreviations continue to be used with regularity in modern English, and not just in scholarly writing. While a lot of Latin abbreviations are rather straightforward (such as etc. for et cetera to mean “and so on” after a list of items), others are apt to be confused, especially if you aren’t aware of what they stand for.

Two abbreviations that are frequently confused even by the most conscientious writers are i.e. and e.g. Not only do they sort of resemble one another, they are used with a level of similarity that can make it hard to keep track which means which. It’s probably not a coincidence that they are among the most commonly looked-up abbreviations in the dictionary.

How to Use 'i.e.'

I.e. stands for id est, or “that is,” and is used in front of a word or phrase that restates what has been said previously. That restatement is meant to clarify the earlier statement:

An examination of the data in Table 1 indicates that all but one of the Council of Governments regions have experienced population growth due to natural increase (i.e., had more births than deaths).
— Nazrul Hoque, The Houston Chronicle, 12 Aug. 2019

Hoque uses i.e. here to clarify what is meant by "natural" population increase.

I.e. is similarly useful for defining or explaining a term or concept whose meaning readers might not know:

If your home has “hard water” (i.e., a high mineral content), your sinks, showers, and tubs no doubt bear white or yellow buildup as a result.
— Melissa Reddigari, BobVila.com, 22 Aug. 2019

...or give greater precision to parameters implied by a term that can mean different things to different people:

Either you're old, like me, and were aghast at the idea of someone defiling your beloved Jump Street. Or you're young (i.e., born after the '87 launch of the TV series that made Johnny Depp a star), and your reaction was closer to, "What's 21 Jump Street?"
— Alynda Wheat, People, 26 Mar. 2012

While i.e. is often set off by brackets or parentheses, it can sometimes follow a comma or em dash. It is usually followed by a comma that sets off the restatement that comes after.

How to Use 'e.g.'

E.g. stands for exempli gratia in Latin and means “for example.” Just like the English phrase, it is used before an item or list of items that stand as an example of the category of thing stated earlier:

These perceptions of harm were powerful enough to influence similar judgments in unrelated contexts: The more immoral people saw a given act to be, the more they saw pain in minor injuries (e.g., hitting your head, cutting your finger) and the more they detected suffering in ambiguous facial expressions.
— Kurt Gray and Chelsea Schein, The New York Times, 30 Jan. 2015

If you already know the specific make, model and year of car you want to buy, searching is easy: Websites like autotrader.com, cars.com, carfax.com and truecar.com round up used cars, typically those being sold at franchise dealerships (e.g., a Honda, Subaru or Ford dealership) and independent auto dealerships (like used car lots).
— Jen A. Miller, The New York Times, 25 Apr. 2019

It should be highlighted here how e.g. functions differently from i.e. In the Gray/Schein quote, hitting your head and cutting your finger are examples of minor injuries—a class that could include other kinds of injuries that are not given as examples, such as a burn or a bruise. The car brands in the Miller quote are examples of the kinds of brands commonly sold in franchise dealerships. If the writer were instead giving a clearer description of what was meant by franchise dealership, then she would use i.e.

Like i.e., e.g. is often, but not always, set off by parentheses. Since both abbreviations are used in similar situations, with similar functions—following a noun or category and preceding another noun or list—the tendency to confuse them is understandable. One mnemonic device that could help keep them straight is to remember that e.g. and example begin with the same letter, while i.e. begins with the same letter as is (found in that is).

It also helps to remember that both that is and for example function in English in the same way as i.e. and e.g., respectively, so that each can be swapped out for its English counterpart:

They had adopted their gender-neutral name a few years ago, when they began to consciously identify as nonbinary - that is [read: i.e.], neither male nor female.
— Amy Harmon, The New York Times, 2 June 2019

There were things that shouldn't work together but did, bringing to mind that only-in-New York funky style (silver pants and a matching silver bag, for example [read: e.g.], and cowboy shoe boots).
— Booth Moore, The Hollywood Reporter, 14 Feb. 2017

https://www.merriam-webster.com/words-at-play/ie-vs-eg-abbreviation-meaning-usage-difference