grammatical number - What is the correct possessive for nouns ending in "‑s"?

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

Your example sentences confuse two different problems.

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

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:

Confucius' sayings

Jesus' teachings

However, this doesn't apply if 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]