On the use of so-called 'zero genitive', marked by a simple apostrophe in spelling ('), as opposed to the 's genitive, Quirk, Greenbaum, Leech and Svartvik specify in A Comprehensive grammar of the English Language (pp. 320 & 321) that: More @Wikipedia
Hover over any link to get a description of the article. Please note that search keywords are sometimes hidden within the full article and don't appear in the description or title.