The most common error in using parenthesis marks (besides using them too much) is to forget to enclose the parenthetical material with a final, closing parenthesis mark. The second most common is to place concluding punctuation incorrectly. The simplest sort of example is one in which the entire sentence is enclosed in parentheses. (Most people understand that the final punctuation must remain inside the closing parenthesis mark, like this.)
More troublesome are sentences in which only a clause or phrase is enclosed in parentheses. Normally a sentence’s final punctuation mark—whether period, exclamation point, or question mark—goes outside such a parenthesis (like this). However, if the material inside the parenthesis requires a concluding punctuation mark like an exclamation point or question mark (but not a period!), that mark is placed inside the closing mark even though another mark is outside it. This latter sort of thing is awkward, however, and best avoided if you can help it.
For some reason, many writers have begun to omit the space before a parenthetic page citation, like this:(p. 17). Always preserve the space, like this: (p. 17).
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