Lest anyone think that this blog is some sort of new-age feel-goodery where everyone’s free to say whatever they want, I’m making sure that my first real grammar post is going to tell you what you should do. This is a mistake that I used to make all the time and only recently did I find out what the real deal was. What’s the difference between i.e. and e.g.? Or, more violently, who’d win the battle of i.e. v. e.g.? Well, it depends on whose home court you’re on.
i.e. comes from the Latin id est, which means it is. In English, its use is basically limited to situations where you would say “that is” or “in other words” or “namely”. It is used when there is only one thing (or set of things) to which you could be referring. e.g. comes from the Latin exempli gratiâ, which means for the sake of an example (see johnwcowan’s comment below for further information on this). Anyway, its use is limited to cases where you’d want to say for example, including, or such as.
In a sense, e.g. is like combining i.e. with etc. for super Latin fun time. i.e. says “I’m going to tell you exactly what I’m referring to (with nothing omitted)”, while e.g. says “I’m going to list some of the things I’m referring to (but there are others, too)”. Using i.e. means there is nothing else that fits your description, while using e.g. implies you’ve left some stuff off and have only stated representative items from the set. Another way to think of it is that if you use “i.e.”, you should be able to replace what precedes i.e. with what follows it: I haven’t figured out the toughest problem, i.e., how to make her love me is approximately the same as I haven’t figured out how to make her love me.
If you’re the sort of person who likes mnemonics, you can think of i.e. as it’s everything, since you didn’t leave anything out, and e.g. as examples go on. Or you can use other mnemonics such as in essence v. examples given.
Some quick additional thoughts: In written American English, it seems to be standard (i.e., I don’t remember ever seeing it another way) to set i.e. or e.g. off from the rest of the sentence with commas. Apparently in the olden days these abbreviations were italicized, but that’s no longer the case. I don’t really use these abbreviations in speech (in part because “e.g.” is really awkward to say), but that’s a matter of personal style.
Some usage examples with prissy marginal notes:
- Perhaps you are thinking of a landlocked South American country, i.e., Paraguay or Bolivia.
- Or maybe a landlocked European country, e.g. Austria, Switzerland, or Moldova.
[There are only two landlocked South American countries, namely the two in sentence 1. Hence "i.e.", it's everything. But there's a lot of landlocked European countries not mentioned in sentence 2, like Andorra or San Marino. Hence "e.g.", examples go on.]
- We all want the same thing, i.e., a tuna-salad sandwich with relatively little mayonnaise.
- We all want the same things, e.g., comfort, care, and power beyond our wildest beliefs.
[Note that in the first sentence there is a single thing that is desired (the sandwich), while in the second there are other things besides comfort, care, and power that we all desire (an obvious untruth).
Summary: enjoy using these abbreviations correctly (i.e., as per this discussion) and you’ll impress the socks off of many Latin-lovers (e.g., Classicists, playwrights, judges, the Pope).