Thread:HRBM/@comment-31096134-20170403020626/@comment-28097343-20170403021248

It's more common knowledge rather than it being present in the Manual of Styles or not.

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 Fumus' angels 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 incorrectly write Mr. Hasting's pen (his name is not Mr. Hasting). Correct: Mr. Hastings' pen