Brackets (or  the Parenthesis.[2])

L. When a clause not strictly belonging to a sentence is thrown in, so to speak, in passing, the clause is enclosed within brackets.

[2]It seems better to use the term "brackets" both for the curved and for the square brackets. "Parenthesis" can then be kept to its proper use, as the name for the words themselves which form the break in the sentence. We may note that in like manner the terms "comma," "colon," "semicolon," originally signified divisions of a sentence, not marks denoting the divisions. "Period" meant a complete sentence; and it still retains the meaning, somewhat specialized.

It is said, because the priests are paid by the people (the pay is four shillings per family yearly), therefore they object to their leaving.

In full confidence of this unalterable truth, I now (quod felix faustumque sit ) lay the first stone of the Temple of Peace.

Over and above the enclosing brackets, a parenthesis causes no change in the punctuation of the sentence that contains it; in other words, if we were to omit the parenthesis, no change ought to be necessary in the punctuation of the rest of the sentence. The comma is inserted after the parenthesis in the first example, because the comma would be needed even if there were no parenthesis.

In the second example, there would be no comma before "lay," if there were no parenthesis; accordingly the comma is not to be inserted merely because there is a parenthesis. A parenthesis is sufficiently marked off by brackets.

Observe also that the comma in the first example is placed after, not before, the parenthesis. The reason for this is that the parenthesis belongs to the first part of the sentence, not to the second.

LI. A complete sentence occurring parenthetically in a paragraph is sometimes placed within brackets.

Godfrey knew all this, and felt it with the greater force because he had constantly suffered annoyance from witnessing his father's sudden fits of unrelentingness, for which his own habitual irresolution deprived him of all sympathy. (He was not critical on the faulty indulgence which preceded these fits; that  seemed to him natural enough.) Still there was just the chance, Godfrey thought, that his father's pride might see this marriage in a light that would induce him to hush it up, rather than turn his son out and make the family the talk of the country for ten miles round.

Note that the full stop should be placed inside, not outside, the brackets.

LII. Where, in quoting a passage, we throw in parenthetically something of our own, we may use square brackets.

Compare the following account of Lord Palmerston: "I have heard him [Lord Palmerston] say that he occasionally found that they [foreign ministers] had been deceived by the open manner in which he told them the truth."

"The Leviathan  of Hobbes, a work now-a-days but little known [and not better known now than in Bentham's time], and detested through prejudice, and at second-hand, as a defence of despotism, is an attempt to base all political society upon a pretended contract between the people and the sovereign."—Principles of Legislation.

To use the square brackets in this way is often more convenient than to break the inverted commas and to begin them again. But in the case of the word sic —where it is inserted in a quotation to point out that the word preceding it is rightly quoted, and is not inserted by mistake—the ordinary brackets are used.

"The number of inhabitants were (sic ) not more than four millions."

Another case may be mentioned in which the square brackets are used: where in the passage quoted some words have been lost, and are filled in by conjecture. Prof. Stubbs quotes from one of the Anglo-Saxon laws:

"If ceorls have a common meadow, or other partible land to fence, and some have fenced their part, some have not, and [strange cattle come in and] eat up the common corn or grass, let those go who own the gap and compensate to the others."