Parentheses (always used in pairs) allow a writer to provide additional information. The parenthetical material might be a single word, a fragment, or multiple complete sentences.
Whatever the material inside the parentheses, it must not be grammatically integral to the surrounding sentence. If it is, the sentence must be recast. This is an easy mistake to avoid. Simply read your sentence without the parenthetical content. If it makes sense, the parentheses are acceptable; if it doesn’t, the punctuation must be altered.
Correct: The president (and his assistant) traveled by private jet.
