| APA | 21. (2012, February 14). In Acronym Geek. Retrieved 06:56, February 14, 2012, from http://www.acronymgeek.com/21 |
| MLA | "21." Acronym Geek. (2012). Acronym Geek. 14 Feb 2012 <http://www.acronymgeek.com/21>. |
| MHRA | '21', Acronym Geek, 14 February 2012, <http://www.acronymgeek.com/21> [accessed 14 February 2012] |
| Chicago | "21", http://www.acronymgeek.com/21 (accessed February 14, 2012). |
| CBE/CSE | 21 [Internet]. Acronym Geek; 2012 Feb 14 [cited 2012 Feb 14]. Available from: http://www.acronymgeek.com/21. |
| Bluebook | 21, http://www.acronymgeek.com/21 (last visited February 14, 2012). |
| Bluebook: Harvard JOLT | See Acronym Geek, 21, http://www.acronymgeek.com/21 (optional description here) (as of Feb. 14, 2012, 06:56). |
| AMA | 21. Acronym Geek. February 14, 2012, 06:56. Available at: http://www.acronymgeek.com/21. Accessed February 14, 2012, 06:56. |