The parenthesis is often used to enclose incidental explanatory matters in a sentence. Normally the sentence is complete without the information in parenthesis which is inserted to add color or explanation but contains information not vital to the meaning of the sentence. Have you ever wondered how to punctuate inside the parenthesis?
If the parenthetical thought is inserted within a sentence before the final punctuation, the first word is not capitalized and usually there is no punctuation inside the parenthesis even though what is inside the parenthesis may be a full sentence:
“I called the police (it wasn’t the first time) because my neighbor’s party went into the early hours.”
However, if the thought inside the parenthesis is either a question or an exclamation, punctuation within the parenthesis should be included:
“I called the police because the noise from my neighbor’s party (is this ever going to end?) was keeping us all awake.
“I called the police (we were panic stricken!) because something had triggered our alarm at 3:30 a.m.”
An interesting rule applies if the parenthesis is within a sentence but contains two full sentences; within the parenthesis the initial word is not capitalized but at the end of the first sentence, there should be punctuation; the first letter of the next sentence should be capitalized, but there should be no punctuation after the second sentence:
“The game was over very late (there was a triple overtime. The winning goal was scored with only three seconds left in the period), and we were all late to work the next day.”
If the parenthesis is outside a sentence, within the parenthesis the sentence or sentences should be capitalized [first word] and punctuation should be added:
“Exhausted from the overnight flight, we could barely drag ourselves into our home. (The cab driver hauled all of our luggage in. Otherwise, we might have left it on the sidewalk.)”
(Note that a parenthetical thought within a sentence need not be a full sentence; though in most cases when the parenthesis is outside a sentence, the thought expressed should be a complete sentence, as here.)
If there is a quotation inside parenthesis, the first word within the quotation should be capitalized, but there is no punctuation at the end unless it is a question or exclamation:
“The police were exasperated (they said “We’re keeping our eye on that crowd”) but gave us a sense of security.”
Not to be totally ignored is the bracket which is more sparsely used than the parenthesis, but at times can serve a vital purpose. One of the most common is to set off a parenthetical thought within parenthesis:
“John arrived early (unexpectedly so [his plane was ahead of schedule] since he had to make two connections), and we were the first to arrive at the meeting.”
Brackets should not be used in place of parenthesis and normally are used to elucidate or clarify.
“The wine cost 24 euros [$28].”
“The president said, ‘My next press secretary is to be Bill [Johnson].'”
Brackets can be used with “sic” to indicate that a quoted passage contains a misspelling or other mistake, and the person who is citing the quotation does not want to be identified with the error.
“She wrote, ‘One of the joys of the Christmas season is that angles [sic] are everywhere.’ ” (She has trouble spelling “angels.”)
It has been said, however, that excessive use of “[sic]” exposes the user as an intellectual snob. But it can be fun (gotcha!), used moderately and properly.
— Ken Butera