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It's not always clear cut---
Rule 1c. Some writers and editors add only an apostrophe to all nouns ending in s. And some add an apostrophe + s to every proper noun, be it Hastings's or Jones's.
One method, common in newspapers and magazines, is to add an apostrophe + s ('s) to common nouns ending in s, but only a stand-alone apostrophe to proper nouns ending in s.
Examples:
the class's hours
Mr. Jones' golf clubs
the canvas's size
Texas' weather
Care must be taken to place the apostrophe outside the word in question. For instance, if talking about a pen belonging to Mr. Hastings, many people would wrongly write Mr. Hasting's pen (his name is not Mr. Hasting).
Correct: Mr. Hastings' pen
Another widely used technique is to write the word as we would speak it. For example, since most people saying "Mr. Hastings' pen" would not pronounce an added s, we would write Mr. Hastings' pen with no added s. But most people would pronounce an added s in "Jones's," so we'd write it as we say it: Mr. Jones's golf clubs. This method explains the punctuation of for goodness' sake.
Jane S 