I'm a native English speaker, and I'm almost 25, and it's only in the last year or so that I've really started to figure this out. I've never formally studied grammar, and usually go with what "looks or sounds right" - all of which is to say that I might be wrong.

However, I'm almost positive that it would, indeed, by Cornelius' cup. The other doesn't look right. ;)

And it's a great way to learn the language. I someone else is more certain than me. ;)

I think it's "Cornelius' hand". It looks correct. Sorry not to be more positive, but I went to school during the period when teaching grammar was unfashionable!

It's true, Cornelius's hand is correct.

I had an English teacher in my senior year of high school who was a stickler for grammar and punctuation. It was a "college prep" class, so she made us write and write and write until we never made a mistake in either. Freshman English in college was a breeze. I've been grateful to this lady ever since.

Actually yannik the above people are wrong actually.

Got the following from http://www.grammarbook.com/punctuation/apostro.asp[/url]

Rule 2. Use the apostrophe to show possession. Place the apostrophe before the s to show singular possession.
Examples: one boy's hat
one woman's hat
one actress's hat
one child's hat
Ms. Chang's house

NOTE: Although names ending in s or an s sound are not required to have the second s added in possessive form, it is preferred.
Mr. Jones's golf clubs
Texas's weather
Ms. Straus's daughter
Jose Sanchez's artwork
Dr. Hastings's appointment (name is Hastings)
Mrs. Lees's books (name is Lees)

So it should 's at the end for singular possession no matter what. Hope this helps you. :)

It helps a lot, thanks! And thanks for the link, it might proove useful.

everyone here is wrong, or right, depending on how you look at it.

English (UK, Australia, New Zealand) use is Cornelius'

American (US Canada etc) is Cornelius's