In rhetoric, a parenthesis (plural: parentheses; from the Ancient Greek word παρένθεσις parénthesis 'injection, insertion', literally '(a) putting in beside') or parenthetical phrase is an explanatory or qualifying word, clause, or sentence inserted into a passage. The parenthesis could be left out and still form grammatically correct text. Parenthetical expressions are usually delimited by round brackets (also known as parentheses), square brackets, dashes, or commas. English-language style and usage guides originating in the news industry of the twentieth century, such as the AP Stylebook, recommend against the use of square brackets for parenthesis and other purposes, because "They cannot be transmitted over news wires." Usage of parentheses goes back (at least) to the 15th century in English legal documents.