A reader wants to know if the abbreviations i.e. and e.g. should be followed by a comma.
i.e.: from the Latin phrase id est, “that is.” Used in English to restate a previous word or expression: “He really enjoys a good bildungsroman, i.e., coming-of-age novel.”
e.g.: from the Latin phrase exempli gratia, “for the sake of the example.” In English, it means “for example” and is used to introduce one or more examples: “I like animals, e.g. dogs, cats, and horses.”
The two terms are frequently mixed up. If you have trouble remembering which means “in other words” and which means “for example,” you can use a mnemonic to keep them apart, or you can avoid using them altogether.
A simple mnemonic that helps many writers is the fact that the word example begins with the letter e. E.g., therefore, is the one that means “for example.”
On the other hand, instead of e.g., you can write “for example,” and for i.e., you can write “namely” or “in other words.”
Style guides do not agree on whether or not a comma should follow both these abbreviations. They do all agree that a comma precedes i.e. when the i.e. phrase occurs in a running text (i.e., not enclosed in parenthesis).
The consensus seems to be in favor of the comma in American usage; against it in British usage.
The Penguin Writer’s Manual (British) shows both i.e. and e.g. without a following comma.
Fowler, in his venerable Modern English Usage, opines that
“whether a comma follows [e.g.] or not is indifferent, or rather is decided by the punctuation-pitch of the writer of the passage.
He says nothing of i.e.
The Chicago Manual of Style states that i.e. and e.g. should be “confined to parentheses and notes and followed by a comma.”
The AP Stylebook, whose “punctuation-pitch” leans generally to the side of “the fewer commas the better,” is pro-comma when it comes to i.e. and e.g. According to AP, both abbreviations are “always followed by a comma.”
As with so many matters of punctuation, the writer’s best practice is to choose a style reference and follow its recommendations.