An apostrophe is used in English to indicate possession (http://encyclopedia.thefreedictionary.com/Possession+(linguistics)). The practice ultimately derives from the Old English (http://encyclopedia.thefreedictionary.com/Old+English+language) genitive case: the “of” case, itself used as a possessive in many languages. The genitive form of many nouns ended with the inflection (http://encyclopedia.thefreedictionary.com/Inflection) -es, which evolved into a simple -s for the possessive ending. An apostrophe was later added by way of a peculiar error as it was thought that the -s was short for "His", as in "King His crown". The apostrophe was introduced to mark the ostensibly omitted "Hi" when this was in fact quite false. Nevertheless the error worked favourably as it marked the actually omitted -e.