<<In general , just add an apostrophe at the end of plural words ending in "s": These are the ladies' rooms>>
It's stupid of me to say that...you all know anyhow...sorry! ;-(
The "rule" I learned in school was to add apostrophe-ess if
the ess if pronounced, and add only an apostrophe otherwise.
Of course not everyone agrees with any one rule.
Since I don't pronounce James' differently than James I don't
write James's (which is pronounced like Jameses). This lack
of pronunciation can only occur on words that end with an
ess sound. Some but not all words that end with that sound
have a silent possessive:
an ounce's worth in gold BUT two days' worth of time
I would never write "two days's worth of time," but the
pattern "two X's worth" always takes a possessive. That
is one place that Strunk and White (see above) are just
wrong. Google give 2250 hits for days's and 122,000,000
Two corrections to my previous post. They are somewhat serious.
1) Google ignores single quotes, so my numbers don't mean much.
There are only 136 hits for "days's worth" and I claim it is
completely nonstandard regardless of google, anyway.
2) One can only ignore the 'es' sound of the plural only if
the previous sound is a _voiced_ s. So ounce's has
to have the 's because the ess sound in ounce is not
voiced, in my accent at least.