What are square brackets and how to use them?
Square brackets ([ ]) are used to set off an interruption or insertion within a direct quotation. They are used to:
• Mark where something has been omitted from the source material and/or mark modifications to it
• Insert explanatory material in the source material
• Insert the word sic in the source material.
Marking omissions from the source material and modifications to it
If you want to quote a particular passage while leaving out a section of it, you can achieve this by inserting an ellipsis (…) to represent the missing section. If you then need to add an extra word or two to link up the pieces of the quotation, you put these extra words inside square brackets to indicate that they are not part of the quotation.
Here is an example taken from Erskine Childers’ The Riddle of the Sands:
I have read of men who… dress regularly for dinner in order to maintain their self-respect and prevent a relapse into barbarism… [A]t seven o’clock… [I was making] my evening toilet in my chambers in Pall Mall…
The first set of square brackets is a modification setting off the replacement of a lower-case a with an upper-case a because it is the start of a new sentence. This is permissible even though a capital a does not appear in the original, but without this minor change the ‘rules’ will be broken. The second set of square brackets sets off inserted words that give the sentence meaning.
The insertion of explanatory material and square brackets
Occasionally you may find it necessary to interrupt a quotation you are citing in order to explain or clarify something. To do this you enclose your comments in square brackets (never parentheses).
On the road [to Aliwal North] we were overtaken by a thunderstorm which lasted for five hours and we were drenched to the skin.
When this passage is taken out of context it is not clear which road the writer is referring to. The inserted words clarify which road it is and the square brackets indicate that the inserted words are not those of the original writer.
Although it is unusual to do so, some authors insert their own initials within the square brackets preceded by a dash. So my insertion above could have been written like this: [to Aliwal North – JD]. Although this isn’t wrong it’s not necessary.
Inserting the word sic in square brackets
If the material you are quoting contains a mistake of some kind – and you want to make it clear to your reader that the mistake is contained in the original material and is not yours – you insert the Latin word sic in square brackets immediately behind the mistake. Sic, meaning ‘thus’, indicates that the ‘mistake’ appears in the original source, and that its reproduction is intentional.
In the sentence below there is a typographical error and we point this out to the reader by inserting the square-bracketed word sic.
The article states that dinosaurs became extinct about 60,000 [sic] years ago.
The same applies when you want to point out that something is not a mistake and it is likely that the reader will think it is a mistake. The extract below refers to a cricket match played over the abnormal duration of nine days.
The famous ‘timeless test’ between England and South Africa in 1939 was abandoned as a draw after nine days of play [sic].
Similarly, when part of a quotation needs to be emphasised this can be done by placing that part in italics and indicating, in square brackets, that you have done so, not the author. So the sentence above could have been written as follows:
The famous ‘timeless test’ between England and South Africa in 1939 was abandoned as a draw after nine days of play [emphasis added].
I have also seen [my emphasis] or [my italics] etc.
Other uses of square brackets
In works of an academic nature square brackets are used for citing references. This is if the so-called number system is being used. In this system a number in square brackets in the text – such as  – draws the reader’s attention to a particular reference in the bibliography. As stated elsewhere, the Harvard System is preferred.
Specialist fields such as mathematics use square brackets but this is beyond the scope of this article.
So, to summarise, square brackets are used to mark omissions, modifications, explanations and the word sic when these have been inserted in original material.