| APA | PEN. (2012, February 14). In Acronym Geek. Retrieved 01:19, February 14, 2012, from http://www.acronymgeek.com/PEN |
| MLA | "PEN." Acronym Geek. (2012). Acronym Geek. 14 Feb 2012 <http://www.acronymgeek.com/PEN>. |
| MHRA | 'PEN', Acronym Geek, 14 February 2012, <http://www.acronymgeek.com/PEN> [accessed 14 February 2012] |
| Chicago | "PEN", http://www.acronymgeek.com/PEN (accessed February 14, 2012). |
| CBE/CSE | PEN [Internet]. Acronym Geek; 2012 Feb 14 [cited 2012 Feb 14]. Available from: http://www.acronymgeek.com/PEN. |
| Bluebook | PEN, http://www.acronymgeek.com/PEN (last visited February 14, 2012). |
| Bluebook: Harvard JOLT | See Acronym Geek, PEN, http://www.acronymgeek.com/PEN (optional description here) (as of Feb. 14, 2012, 01:19). |
| AMA | PEN. Acronym Geek. February 14, 2012, 01:19. Available at: http://www.acronymgeek.com/PEN. Accessed February 14, 2012, 01:19. |