Before venturing any further into the exotic realm of possessive nouns, note a basic point: Do not confuse simple plurals with possessive nouns. If we want to change dog to plural so as to designate more than one dog, we add s – nothing else, just s. We do not use the apostrophe to make words plural.*
Note also that, if the phrase already contains the words of or for to show possession, one does not add the possessive 's or apostrophe ending. For instance, if one writes, "The names of the two dogs are Ralph and George," one does not need an apostrophe on or in the word dogs. The phrase "of the two dogs" already states the possession. The word dogs is not possessive but a simple plural. Grammatically, it is the object of the preposition of, and, as such, it is not possessive.**
Rules for Forming the Possessive
Form the possessive of nouns not ending in s by adding an apostrophe plus the letter s.
> the dog's tail . . . the children's toys . . . his master's voice . . . the company's employees . . . the men's room . . . a week's wages.
Form the possessive of nouns that end in s by adding just an apostrophe.
> both boys' parents . . . the two dogs' barking . . . these students' grades . . . several companies' products . . . the ladies' room . . . Moses' laws. (But read on regarding the last example and the rule for singular nouns ending in s.)
It is as simple as that. However, we do have one area of particular difficulty – the matter of singular nouns ending in s. Here, the usual rule is to add apostrophe + s ('s) to the s-ending singular as long as pronouncing the extra syllable (which sounds like "iz" or "ziz") is not awkward. Thus, we have: James's house, Tess's car, and the boss's office. Some s-ending singular nouns, however, become awkward to say if we add "iz" or "ziz" to them. Try sounding out "Jesus(iz) teachings," "Moses(iz) laws," or "Sophocles(iz) plays." It's tough. Thus, in these instances, we use only the apostrophe: Jesus' teachings, Moses' laws, or Sophocles' plays. In such instances, though, we should try to avoid using the possessive case altogether and write: the teachings of Jesus, the laws of Moses, the plays of Sophocles.
Some constructions can be tricky. One awkward area is the use of back-to-back possessives. One might, for instance, want to refer to the children belonging to the two brothers of my father (i.e., my cousins), and, unless one wrote "my cousins," this would require a double possessive: my father's brothers' children. Ouch! We better stick with "cousins."
Another more common problem is the use of a possessive form when literal ownership is not involved. Here, we are still using the possessive shorthand as a substitute for the "of" or "for" phrase even though the situation does not involve true possession. Usually, this occurs with sums of money or periods of time. For instance, we refer to the wages for a week as "a week's wages." (That is logical; one cannot correctly make "a weeks" possessive because there is no such thing as a weeks.) And the worth of a dollar is a dollar's worth. Here one needs to be very careful not to err with words ending in s – a vacation of two months is two months' vacation; the worth of two dollars is two dollars' worth.