Basic rules:

1. Apostrophes mark where letters have dropped out of a contraction, such as don't for do not or it's for it is.
2. Apostrophes mark the possessive form of nouns:
   • singular nouns normally add -s after the apostrophe to form their possessives: bird's means belonging to a bird, octopus's means belonging to an octopus.
   • plural nouns use the apostrophe after their final -s: animals' means belonging to more-than-one animal, creepy-crawlies' means belonging to more-than-one creepy-crawly.
   • Greek names ending in -es pronounced "-eez", like Socrates, form their possessives with just a final apostrophe: Socrates'. So too does Jesus form the possessive Jesus'.
   • If you look up "apostrophe (2)" in the Oxford English Dictionary (Loyola has it on-line: go through the Libraries' index by clicking OED Online), you will find that this usage really reflects a contraction too: the possessive form of many nouns used to be written -es, and starting in the 18th century English has stopped using the -e- and substituted the apostrophe; even nouns that used to use just -s for their possessive forms are now given the apostrophe too, for consistency's sake.
3. Possessive pronouns don't use apostrophes: ours, yours, his, hers, its, theirs.
4. Plurals also don't use apostrophes.
   • Numerals and names consisting of capital letters are not exceptions - how else would you be able to distinguish possessives of these forms? the 1960s, IBM's PCs [plural, possessive+plural].

Revised 11 February 2010 by jlong1@luc.edu
http://www.luc.edu/classicalstudies/