Possessive form for a surname ending with "z"

Question

What is the proper possessive form for a surname that ends with “z”? Is it z’ or z’s?

Answer 1

Spelling Possessives: A simple rule with zero exceptions

There is no special rule for surnames which does not also apply to common nouns.

For that matter, there is no special rule for singulars versus plurals, either. The possessive form is the same no matter how the word is spelled or what its number is or what its capitalization is, because it follows from the pronunciation not from the spelling.

There’s just one universal rule at work here, one with no exceptions, for forming possessives. As I write in my related answer to the question Which singular names ending in “s” form possessives with only a bare apostrophe?

The most useful rule — and the most general and the easiest to remember — is simply that you add ’s whenever you actually say an extra /əz/ at the end when forming the possessive, compared with how you say the non-possessive version. Let your own ear be your guide. That’s all there is to it. No fancy rules full of exceptions. Just your own ear (as a native speaker, mind you).

This is true no matter how the word is spelled. It’s a sound law not a spelling rule, and as such has no exceptions due to spelling. Here are the only two “rules” you need ever remember to form the possessive in English:

1. To form the possessive of anything,* add an apostrophe.
2. Then if and only if you say an extra “s” sound of some sort compared with how you pronounce the non-possessive form, and an ‹s› after the apostrophe.

I cannot overstress that it does not matter how it’s spelled to start with:

• Spelling does not matter.

• Capitalization does not matter.

This is about sounds, not about orthography. The only thing that matters is how it’s said, and it does not matter how you pronounce that ‹s›, provided it is not silent. It may be phonemic /s/ or /z/ or /əz/; you always spell it ‹s›.

Said with an extra -s

• This is that waltz’s problem.
• This is Fritz’s problem.
• This is Bill Seitz’s problem.
• This is the quiz’s problem.
• This is Jezebel’s problem.
• This is Jez’s problem.
• This is the Prez’s problem.
• This is Cruz’s problem.
• This is Jess’s problem.
• This is Jessie’s problem.
• This is Juan Gómez’s problem.
• This is Cristovão Conçalves’s problem. (But you might not add an /s/ sound; if not, don’t add it in spelling.)
• This is Joan’s problem.
• This is the Queen of England’s problem.
• This is Elsa’s problem.
• This is someone else’s problem.
• This is Clarice’s problem.
• This is my niece’s problem.
• This is Carolina’s problem.
• This is that corpus’s problem.
• This is those corpora’s problem.
• This is that genius’s problem.
• This is that genus’s problem.
• This is those genera’s problem.

But assuming you don’t add an /s/ to make these in their possessives when you say them (almost no one does), they only get a bare apostrophe not an extra letter:

Said without an extra -s

• This is those waltzes’ problem.
• This is the Gomezes’ problem. (English version)
• This is those quizzes’ problem.
• This is the Joneses’ problem.
• This is both my nieces’ problems.
• This is the Carolinas’ problem; Georgia has nothing to do with it.
• This is Mercedes’ problem.
• That was Socrates’ problem.
• That was the Kingdom of the Two Sicilies’ problem.
• This is that series’ problem.
• This is those series’ problem.
• This is that species’ problem.
• This is those species’ problems.
• He did it for conscience’ sake.
• Oh for Heaven’s sake, just spell it like you say it and now it’s nobody’s problem!
• Oh for goodness’ sake, just spell it like you say it and now it’s nobody’s problem!

The corollary to this is that if you do happen to be one of those rare people who actually do add an /s/ sound to one or another from the previous list, then you would have to spell it that way, too.

But not many people actually do say those with an extra /s/.

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Footnotes

• Anything but a personal pronoun, that is.

Answer 2

Although an apostrophe is what makes a possessive, it is an issue of style whether to add an s at the end of a noun ending with s or z.

By The Chicago Manual of Style:

Jesus's followers

but from Catechism of the Catholic Church:

Jesus' followers