What is the correct possessive for nouns ending in “‑s”?

Question

What is the possessive of a noun ending in ‑s? Are these both right, or is the second one wrong?

1. the boys' books

2. the boss' car

Answer 1

Your example sentences confuse two different problems.

For nouns that are plural (such as "boys"), the possessive formed in writing by adding an apostrophe after the plural -s. This is pronounced the same as the plural and the singular possesive:

The boys' books [boys' sounds like boys]

For singular nouns that end in -s, the possessive is formed by adding -'s, just as with other nouns. This is pronounced as if the spelling were -es:

The boss's car. [boss's sounds like bosses]

There is a partial exception for proper names that end in -s. These names sometimes form their possessive by simply adding an apostrophe, and without changing their pronunciation. Thus, we often see:

Confucius' sayings

Jesus' teachings

However, this doesn't apply when the name ends with a letter other than s, even if it's pronounced with an s. These names form their possessive as normal:

Marx's theories

In the opposite case of a name which ends in a silent s, the possessive is usually formed by adding an apostrophe in writing, but the apostrophe causes the silent s to be pronounced:

Camus' novels [the final -s in Camus is not silent here]

Answer 2

On singular nouns that end with an "s" or "z" sound, Wikipedia has a say. According to the article, there is no hard and fast rule on this one and different "authorities" prefer different styles.

See also St. James's park and St. James' park.

Answer 3

As far as I can remember being taught, it is acceptable but not required to omit the "s" after apostrophe for the possessive of a noun ending in /s/ or /z/. "The boss' car" and "the boss's car" are both common and correct, although the former might seem a bit old-fashioned to some people, or simply represent the omission of an unnecessary syllable in rapid speech, depending on the register. I don't hesitate to use both forms interchangeably in speech, and I tend to omit the extraneous "s" in writing.

There is at least one fixed expression

... for conscience' sake ...

which would sound awkward and contrived to me if an extraneous possessive "s" were included after the apostrophe.

This is definitely a situation to use the form that sounds the most natural the way one would say it aloud, and cheerfully ignore the prescriptivists, except that the following is just not correct:

... the boys's car ...