You might be surprised to learn that the abbreviations i.e. and e.g. mean two different things. Many people use them interchangeably; in fact they are not interchangeable.
As I set out to write this blog, I found agreement about the following:
e.g. means “for example” and comes from the Latin exempli gratia; whereas i.e. means “that is” and comes from the Latin id est.
Grammar Girl sugests some great tricks to remember these definitions. She suggests thinking “egg” for e.g., as in “eggsample” or just remembering that it starts with an e for “example.” For i.e. she suggests thinking “in essence” or just remembering that it starts with i for “in other words.”
In terms of usage, here are some examples:
There are 7 colors in the rainbow, i.e., red, orange, yellow, green, blue, indigo and violet. (complete list)
There are 7 colors in the rainbow, e.g., red, orange and yellow. (an incomplete list)
(Use i.e. when you are stating the 7 colors in another way; use e.g. when you are giving examples of them.)
Sweets are her Achilles’ heel, i.e., her weak spot.
Sweets are her Achilles’ heel, e.g., she eats a pint of ice cream if it’s in front of her.
(Use i.e. when you are defining the first part of the sentence in another way; use e.g. when you are giving an example of the first part of the sentence.)
The soldier went AWOL, i.e., Absent Without Official Leave.
Acronyms are words where each letter stands for a word, e.g., AWOL which means “Absent Without Official Leave.”
(Use i.e. when you are explaining the definition; use e.g. when giving an example.)
There are a few things that grammar gurus do NOT agree upon. One is whether i.e. and e.g. need to be italicized, as most Latin abbreviations are. Most agree, however, that because these abbreviations are so common there is no need to italicize them.
[Note: I am italicizing i.e. and e.g. in sentences such as this as an alternative to using quotation marks; in my examples they are not italicized.]
There is disagreement as well as to whether i.e. and e.g. can ever be capitalized, i.e., I.e. or E.g. I would capitalize them if they started a sentence, but not everyone agrees that that’s acceptable.
Should there be a comma after the second period in i.e. and e.g.? Once again there is no agreement on this point. Most sources but not all recommend a comma; in British English a comma is less often used.
Technicalities aside, I recommend that you take a moment to think the next time you write an e.g. or i.e. in a document. You’re more likely to say what you mean.
Questions? I’m happy to answer.