The reason for not citing, for example, (screen 12) is that it may be quite inaccurate to indicate such a screen number for a document printed from the Internet. A screen of displayed text is not equivalent to a printed page from a book or a magazine. Unlike printed material where page numbers are clearly indicated, page and screen numbers on a Web page may vary considerably from one user to another depending on numerous variables such as the size of the monitor used, the user’s choice of font size and font type, setting of pixels, printing using portrait or horizontal/landscape format, choice of paper size, user’s option to suppress graphics or images, selection of number of lines per page, setting of top and bottom as well as left and right margins, the particular browser used as well as the version of the browser used, and other variables, may all have an impact on the outcome of the printout.
1540s, "words, clauses, etc. inserted into a sentence," from Middle French parenthèse (15c.), from Late Latin parenthesis "addition of a letter to a syllable in a word," from Greek parenthesis , literally "a putting in beside," from parentithenai "put in beside," from para- "beside" (see para- (1)) + en- "in" + tithenai "put, place," from PIE root *dhe- "to put, to do" (see factitious ). Sense extension by 1715 from the inserted words to the curved brackets that indicate the words inserted. A wooden parenthesis; the pillory. An iron parenthesis; a prison. ["Dictionary of Buckish Slang, University Wit and Pickpocket Eloquence," London, 1811]