http://owl.english.purdue.edu/owl/resource/621/1/
http://writing2.richmond.edu/writing/wweb/possessive.html
http://www.writing.utoronto.ca/images/stories/Documents/plurals-and-possessives.pdf
http://web.uvic.ca/wguide/Pages/SentApostYes.html
http://web.uvic.ca/wguide/Pages/SentApostNo.html
http://web.uvic.ca/wguide/Pages/SentApostInOb.html
http://web.psych.washington.edu/writingcenter/writingguides/pdf/apostrophes.pdf
http://www.sdc.uwo.ca/writing/handouts/The%20Possessive%20Apostrophe.pdf