Parenthesis marks come in pairs: the plural is spelled “parentheses.” Parentheses look like this: ( ). When you use parentheses to set off material in a sentence, you say that the material is “in parenthesis.” Put something in parentheses if it's a comment, an afterthought, or additional information that is possibly interesting but not essential to the subject. You can also describe something as a parenthesis, like a digressive story about horses in the middle of a political speech.