9 March 2015
In citations to published works, parentheses are used to set off "publication data." When citing a journal or magazine article, we put the date of the issue in parentheses. When citing a book, we use parentheses to set off the place of publication, date of publication and, ideally, the publisher. These basics apply to both printed publications and online publications.
In everyday writing, we do use parentheses for all sorts of "asides." We even refer to the practice as “making parenthetical statements." We add explanations, in parentheses, that would otherwise interrupt the flow of the narrative.
The parenthetical aside, however, is a convention of narrative writing, not citations. In citations, when we need to add comments about the nature of the source or our analysis of its content, etc., those additional remarks go in the comment field at the end of the citation, which always falls outside the parentheses.
PHOTO CREDIT: Adapted from "Pressure Sign Blank Paper," Presenter Media (http://www.presentermedia.com/index.php?target=closeup&id=9238&categoryid=115&maincat=animsp : downloaded 6 March 2015), item 9238; used under license.