Reader John B. Moss asks if there are guidelines for the use of brackets. There are indeed. Academic style guides such as the MLA Handbook for Writers of Research Papers go into such matters at great length.
The most common use of brackets is to enclose explanatory matter that one adds in editing the work of another writer. They indicate that some kind of alteration has been made in the original text.
1. In quoting a passage it is often necessary to insert information that was provided elsewhere in the original text:
I don’t care what he [Poe] meant by it, the line sounds great but makes no sense.
2. Sometimes a word in the quotation is archaic or used in a sense that may not be familiar to the intended reader so the editor may wish to provide an explanation in brackets:
Paul said he was “let [hindered] hitherto.”
In this episode of C.S.I. her character says “Gimme some bling [gaudy jewelery].”
3. Sometimes it’s necessary to change the original capitalization or provide a word in order to make a quotation fit grammatically into the new text:
He was an out-spoken old curmudgeon.
According to Jones’s biographer, “[h]e was an out-spoken old curmudgeon.”
4. Sometimes brackets are used to enclose the dots that indicate missing words. The usual way to indicate that some words have been left out (an ellipsis) is to mark the spot with three dots (…).
It little profits that an idle king,
By this still hearth, among these barren crags,
Match’d with an aged wife, I mete and dole
Unequal laws unto a savage race
According to Ulysses in Tennyson’s poem, “it little profits…an idle king…to…mete and dole unequal laws unto a savage race.”
Some (but not all) academic writers would enclose the dots in a quotation in brackets:
According to Ulysses in Tennyson’s poem, ” it little profits[…]an idle king[…]to[…]mete and dole unequal laws unto a savage race.”
Another use of brackets
Brackets can be used in the context of one’s own writing when more than one thing needs to be set apart. For example:
Watching a popular actor who usually plays good characters play a villain (like Tom Hanks in Road to Perdition ) has a negative effect on many movie-goers.