Baugh 2005. p. 45. - Baugh, L. Sue (2005). Essentials of English Grammar: The Quick Guide to Good English (3rd ed.). New York: McGraw Hill. ISBN 978-0-07-145708-8.
Research and Education Association 2009. p. 177. - Staff of REA (2009). REA's Handbook of English Grammar, Style, and Writing. Research and Education Association. ISBN 978-0-87891-552-1. https://archive.org/details/reashandbookofen00rese_0
University of Chicago Press 1982 Chicago Manual of Style. p. 133. - University of Chicago Press (1982). The Chicago Manual of Style: For Authors, Editors, and Copywriters (13th Revised and Expanded ed.). Chicago and London: University of Chicago Press. ISBN 978-0-226-10403-4. https://archive.org/details/chicagomanualofs00univ_0
Lutz and Stevenson 2005. p. 199. - Lutz, Gary; Stevenson, Diane (2005). The Writing Digest Grammar Desk Reference. Cincinnati, OH: Writer's Digest Books. ISBN 978-1-58297-335-7.
Loberger and Shoup 2009. p. 158. - Loberger, Gordon; Shoup, Kate (2009). Webster's New World English Grammar Handbook (2nd ed.). Hoboken, NJ: Wiley Publishing. ISBN 978-0-470-41080-6.