(while I'm writing this, I'd like to ask: in English is it possible to put the 's even if the last letter is a S ? I can write Michael's posts but is it the same with Chris's posts?)
Opinions vary, but the oft-cited
Chicago Manual of Style's rule is to add apostrophe-s to singular nouns regardless of whether they end in s — thus,
the horse's mouth,
a bass's stripes,
Chris's posts — and add an apostrophe alone to plural nouns unless it's an irregular plural that doesn't end in “s” — thus
puppies' paws,
children's literature.
Their rules do include several exceptions, and they end up saying, “Those uncomfortable with the rules, exceptions, and options outlined above may prefer the system, formerly more common, of simply omitting the possessive
s on all words ending in
s — hence ‘Dylan Thomas' poetry,’ ‘Maria Callas' singing,’ and ‘that business' main concern.’ Though easy to apply, that usage disregards pronunciation and thus seems unnatural to many.”
