Exceptions to the rule of adding an apostrophe “s” for the possessive form are the possessives of ancient proper names ending in “s.” Examples: Moses’ Law, Jesus’ parables, Euripides’ tragedies, Xerxes’ armies.
To avoid an awkward appearance, an apostrophe without an “s” may also be used for the possessive of singular words and names ending in an unpronounced “s.” Examples:
Albert Camus’ novels, Descartes’ three dreams, Vaucouleurs’ assistance to Joan of Arc.
For … sake expressions traditionally omit the “s” when the noun ends in an “s” or “s” sound. Examples: for righteousness’ sake, for goodness’ sake, for Jesus’ sake.
Closely linked nouns are considered a single unit in forming the possessive when the entity possessed is the same for both. Only the second element takes the possessive form. Example: my aunt and uncle’s house. When the entities are different, both nouns take the possessive form. Example: my friends’ and neighbors’ children.