What is the correct syntax for a plural possessive of a word already ending in s? [duplicate]

Possible Duplicate:
What is the correct possessive for nouns ending in s?

Before you vote to close as a duplicate, note that these two questions deal with similar issues to this, but none of them address all three criteria of this question:

• The singular already ends with an s.
• I am dealing with multiple of the thing.
• I want the possessive of all of them in general.

The singular is class, and the plural is classes. The singular possessive is class's (as addressed here).

What is the plural possessive? Is it classes's, or classes'?

Solution 1:

It is classes', which sounds the same as the singular class's.

I would have thought this was a general rule for plural nouns ending -s', -es' or -ies'.

Solution 2:

Here's the rule: For plural nouns that end in S, only add the apostrophe. For singular nouns, add apostrophe-S.

This gives you the following. The class (singular noun): The class's project. The classes (plural noun): The classes' project.