| APA | 21C. (2012, February 10). In Acronym Geek. Retrieved 17:14, February 10, 2012, from http://www.acronymgeek.com/21C |
| MLA | "21C." Acronym Geek. (2012). Acronym Geek. 10 Feb 2012 <http://www.acronymgeek.com/21C>. |
| MHRA | '21C', Acronym Geek, 10 February 2012, <http://www.acronymgeek.com/21C> [accessed 10 February 2012] |
| Chicago | "21C", http://www.acronymgeek.com/21C (accessed February 10, 2012). |
| CBE/CSE | 21C [Internet]. Acronym Geek; 2012 Feb 10 [cited 2012 Feb 10]. Available from: http://www.acronymgeek.com/21C. |
| Bluebook | 21C, http://www.acronymgeek.com/21C (last visited February 10, 2012). |
| Bluebook: Harvard JOLT | See Acronym Geek, 21C, http://www.acronymgeek.com/21C (optional description here) (as of Feb. 10, 2012, 17:14). |
| AMA | 21C. Acronym Geek. February 10, 2012, 17:14. Available at: http://www.acronymgeek.com/21C. Accessed February 10, 2012, 17:14. |