The abbreviations ‘e.g.’ and ‘i.e.’ are common in academic writing, and they can be useful in many contexts. They each have a very specific and different meaning, but people often get them confused. This article explains where each abbreviation comes from and how they should be used.
What do they actually mean?
Academic English – and indeed English in general – tends to be scattered with words and phrases that have been ‘borrowed’ from other languages. In this case, ‘e.g.’ and ‘i.e.’ are abbreviations of Latin words. But what do they mean? To summarise:
- ‘e.g.’ is short for exempli gratia, which translates as ‘for the sake of example’
- ‘i.e.’ is short for id est, which literally means ‘that is’.
As to why we use abbreviations of an ancient language instead of their English equivalents, that is perhaps a question best left to further academic study.
How and when to use ‘e.g.’
If you are introducing an example or a list of examples, then you should use ‘e.g.’. You can use this anywhere you would normally use the English phrase ‘for example’, such as:
Rare-earth elements, e.g., neodymium, can be used to make powerful magnets.
In the above sentence, note that ‘neodymium’ is only one of the possible rare-earth elements that could have been given as an example; it could be replaced with another element or indeed a list of several of these elements. This is an important point.
How and when to use ‘i.e.’
In contrast, if you are indicating one specific thing, then you should use ‘i.e.’. As noted, this abbreviation literally means ‘that is’. However, it might be easier to understand if you think of it as meaning ‘in other words’. For example:
Herbivorous mammals, i.e., those that feed exclusively on plants, tend to be preyed upon by larger carnivores.
In this sentence, ‘herbivorous mammals’ and ‘those that feed exclusively on plants’ mean the same thing but are stated in different words. To be clear, what is given after the ‘i.e.’ is not just an example, it is a re-stating of the same thing in a different way.
Should you include a comma after these abbreviations?
In the two examples above (presented here again for ease of reference), note that both abbreviations are followed by a comma.
Rare-earth elements, e.g., neodymium, can be used to make powerful magnets.
Herbivorous mammals, i.e., those that feed exclusively on plants, tend to be preyed upon by larger carnivores.
Different journals and style guides have different opinions on whether these commas should be present. In broad terms, most US English style guides (and therefore many international journals) require this comma, while many British style guides do not. If in doubt, it is probably better to include a comma.
Also, it is always a good idea to check the style guide and preferences of any journal you are submitting to as you are preparing your manuscript; most should indicate whether they follow US or British English.
Summary
Here are the two simple rules you need:
- If you are giving an example or a list of examples, use ‘e.g.’;
- If you are expressing the same thing in different words, use ‘i.e.’.
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