"I.e." and "e.g." don't mean the same thing.
|Chicago Manual of Style||A comma is usually used after i.e. and e.g.|
|Blue Book of Grammar and Punctuation||Commas are preferable/optional after the abbreviations.|
|The Columbia Guide to Standard American English||[Editors] require a comma after the second period [in these abbreviations].|
|The Guide to Grammar and Writing||The comma [following i.e. and e.g.] makes good sense.|
|Lynch Guide to Grammar||Both abbreviations should be followed by a comma.|
|Fowler's Modern English Usage||Commas do not usually follow i.e. (No comment on e.g.)|
Nevertheless, even though I prefer the comma and have sources to back me up, they almost all use hedge words like “usually” and “preferred.” I've also been told that the commas are used less frequently in Britain, and the only style guide I found that advised against commas was Fowler's Modern English Usage, which has its roots in British English. The bottom line is that in American English, I recommend using a comma after i.e. and e.g. You could probably make an argument for leaving it out in some cases, but do so at your own risk. My personal rule is to use a comma every time.
Finally, I tend to reserve i.e. and e.g. to introduce parenthetical statements, but it's also perfectly fine to use i.e. and e.g. in other ways. You can put a comma before them, or if you use them to introduce a complete sentence that follows after another complete sentence, you can put a semicolon before them. You can even put an em dash before i.e. and e.g. if you are using them to introduce something dramatic. They're just abbreviations for words, so you can use them in any way you'd use the words in essence or for example.
I like fun examples, so here are some extras that didn't make it into the show.
1. Our pet, Squiggly (i.e., the snail we brought home after the lab experiments were finished), loves to curl up on his little patch of grass.
2. Our pet snail, Squiggly, loves vegetation (e.g., grass, leaves, twigs).
Download a Chapter from Grammar Girl's Book
"I.e." versus "e.g." is just one of the many confusing word choices that Mignon Fogarty covers in the "Dirty Words" chapter of her book, Grammar Girl's Quick and Dirty Tips for Better Writing. You can download the chapter by clicking here.
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