Ordinarily, square brackets aren't used very often, except in dictionaries. A detailed dictionary will often use brackets to show the etymology, or the history, of the word being defined. (Now be honest — you've never noticed brackets in dictionaries, have you?)

One use of square brackets is to make certain that quoted material is clear or understandable for readers. Suppose you're quoting a sentence that contains a pronoun without its antecedent, as in this example:

Just who is he? Unless the previous sentences had identified him, readers wouldn't know. In that case, you'd use square brackets this way:

Here's another example:

Readers would have no idea what it was. An announcement of retirement? an unexpected large check? a stripper popping out of a cake?

To explain the pronoun so that readers understand the material more clearly, you might use brackets in this way:

Along the same lines, you use brackets to alter the capitalization of something you're quoting so that it fits in your sentence or paragraph. For example:

Use brackets for quoted material only if their use doesn't change the meaning of what's being quoted.

Another time that brackets are used occurs even less frequently. If you need to give information that you'd normally put in parentheses — but that information is already in parentheses — use brackets instead. This may sound confusing, but take a look at this and you'll see how the rule applies:

Normally, you put a person's birth and death dates in parentheses, but since those dates are placed in material that's already in parentheses, you use brackets instead.

Depending on the type of writing you do, you might add the Latin word sic to the information that you're quoting. You don't know what sic means? Sic shows that what you're quoting has a mistake that you're copying. By seeing the sic designation, readers know that the mistake was made by the original author and not you. Look at this sentence:

Now, you know and I know that “thirty days hath September” — not thirty-one, as stated in the example. By using [sic] readers can tell that you copied the mistake as it was written in the original form. Note that sic is enclosed in brackets (many handbooks or style guides dictate that it be italicized as well).

Most style guides allow you to use either brackets or parentheses to let readers know that you've added italics to quoted material. The only rule is that you keep using the same choice of punctuation throughout the manuscript. Take your pick:

Generally speaking, you'll use brackets rarely — unless you're writing in a particular style. As with any writing, if you're told to use a particular style guide (say, for instance, The Chicago Manual of Style), consult it for the other infrequent times that brackets are used.