Conventions of Composition Rule 16

Rule: Parenthetical words, phrases, or clauses (such as I believe, they say, we know, it has been said, etc. when used NOT at the beginning of a sentence) need to be separated from the rest of the sentence with a pair of commas, pair of em-dashes, or a pair of parentheses. If commas do not instantaneously indicate the parenthetical nature of the word, phrase, or clause, use a pair of dashes or parentheses.

Notes on dashes: Omit the second of the two dashes before a semicolon, colon, period, question-mark, or exclamation-mark. Omit a comma before or after a dash.

Notes on parentheses: Follow the second parenthesis with whatever mark would be needed if the whole parenthetical expression were removed. If a complete sentence is enclosed in curves, capitalize the sentence and place the period, question-mark, or exclamation-mark inside the second curve.

Examples:

Correct: The Vikings, we know, were brave men. (The "we know" is inessential to the sentence's meaning.)

Correct: The Vikings were brave men, we know. (same explanation as above)

Correct: We know the Vikings were brave men. (Here, the "we know" is the subject and verb.)

Correct: Investments of that sort—I speak from years of experience—are always attractive to the general public.

Correct: If you go (and you really ought to go), I trust you will succeed.

Practice deciding if and where to put commas, dashes, and parentheses:

1. I bought and you're not going to believe the great deal I got a new truck.
2. She swam swimming provides an amazing full-body workout to and from work each day.
3. By the time the bonfire started to smolder ten hours after most people had already left the event, we were tired.
4. Skaters do you wear figure or hockey skates usually love when black ice covers the lake.

Resources for further explanation of how to punctuate parentheticals:

Grammar Book's Parentheses and Brackets

The Punctuation Guide's Em Dashes

Writing Forward's Punctuation Marks: Parentheses