Would it be Davis or Davis's?
According to Grammarbook.com, the nerds of the world will argue heatedly on the subject for eternity, but the most roundly accepted rule is to include the apostrophe, along with an extra “S.” (Davis's rather than Davis').
Names are proper nouns, which become plurals the same way that other nouns do: add the letter -s for most names (“the Johnsons,” “the Websters”) or add -es if the name ends in s or z (“the Joneses,” “the Martinezes”).
...
With names, apostrophes are for possessives.
- The Joneses' dinner was a success.
- The Foxes' house was beautiful.
- The Alvarezes' grandmother was delighted.
- The Churches' singing was heavenly.
- The Ashes' train derailed in the mountains.
If you want it boiled down to something simpler, remember this one thing: NO APOSTROPHES. Got it? NO APOSTROPHES. Not in your own name, either: sign it “With warm holiday wishes from the Jameses,” not “the James's” or “the James' ” or “the Jameses',” or, heaven help us, “The Jame's.” Why?
If the name ends in s, z, ch, or sh, you need to add es. That means the Davis family becomes the Davises, the French family becomes the Frenches, the Hernandez family becomes the Hernandezes, and the Glaves family becomes the Glaveses.
And if you're talking about something that belongs to the Davis family you write "the Davises' house."
Jones = Mr. Jones's. Some people favor adding only an apostrophe to a singular noun ending in s, but if you follow the rule, you can't be wrong. If a plural noun does not end in an s, you must make it possessive by adding an apostrophe and an s: women's; children's.
Don't use an apostrophe to make your last name plural. Apostrophes can be used to show possession—à la the Smithsʼ house or Tim Johnsonʼs pad— but they don't indicate there's more than one person in your family.
First, make the noun Williams into a plural: Williamses. Then add the possessive apostrophe according to the rules that gave us “the cats' tails.” That gives us “We had dinner at the Williamses' house last year.”
All you need is an “s” at the end of the name (Smiths, Johnsons). If you have trouble remembering whether the apostrophe is necessary, think of your message. For instance, if you mean to say, “The Smiths live here,” then you don't need the apostrophe on the sign or that sentence.
Do you use singular or plural with family?
In American English, "family" will almost always be used with a singular verb. In British English, it may be used with a singular or a plural verb depending on whether the speaker feels that "family" is being described as a unit or as a group of individuals.
Apostrophes are used to show possession. Your family last name on its own is not possessive. You would say We are the Smiths. You would not say We are the Smith's House.

To show possession when a noun ending in y becomes plural, write ies'. Do not write y's. Exception: Names and other proper nouns ending in y become plural simply by adding an s. They do not form their plurals with an apostrophe, or by changing the y to ies.
Which is correct, Chris's chair or Chris' chair? James's car or James' car? Actually, both ways are correct. If a proper name ends with an s, you can add just the apostrophe or an apostrophe and an s.
For showing family possession with surnames that are plural and possessive, make the name plural first by adding an “s” and then add an apostrophe to make them possessive. The Smiths' car was parked illegally.
If the compound noun (e.g., brother-in-law) is to be made plural, form the plural first (brothers-in-law), and then use the apostrophe + s. Rule 4a. If two people possess the same item, put the apostrophe + s after the second name only. Example: Cesar and Maribel's home is constructed of redwood.
- GENERAL RULE. Simply add an s.
- EXCEPTION. If your name ends in s, x, z, ch, or sh, add an es.
- EXAMPLES. Love, the Andersons. Love, the Joneses. Love, the Smiths. Love, the Foxes. Love, the Johnsons. Love, the Kennedys. Love, the Churches. Love, the Days.
To show possession of a whole family: Add -es or -s to write the family's last name in plural form. Add an apostrophe at the end to show possession.
An apostrophe is needed after the final s to show possession (i.e., the Joneses own the house). An apostrophe is not needed with Mrs. Jones because there is no ownership.
When you're talking about more than one, you first form that plural by adding -ES. One Thomas, two Thomases. Then, to note that something is owned by more than one Thomas, just take the plural and make it possessive: Thomases'.
Should it be Smiths or Smith's?
All you need is an “s” at the end of the name (Smiths, Johnsons). If you have trouble remembering whether the apostrophe is necessary, think of your message. For instance, if you mean to say, “The Smiths live here,” then you don't need the apostrophe on the sign or that sentence.
Jones = Mr. Jones's. Some people favor adding only an apostrophe to a singular noun ending in s, but if you follow the rule, you can't be wrong. If a plural noun does not end in an s, you must make it possessive by adding an apostrophe and an s: women's; children's.
Add es to your last name. Examples: If your last name is Jones, you will change it to Joneses. If your last name is Davis, you will change is to Davises.
The “Chicago Manual of Style” says that you form the possessive of proper names ending in S the same way you handle plain-old nouns: Add apostrophe and S. The boss's house. Ross's house.
Don't use an apostrophe to make your last name plural. Apostrophes can be used to show possession—à la the Smithsʼ house or Tim Johnsonʼs pad— but they don't indicate there's more than one person in your family.
When making your last name plural, you don't need to add an apostrophe! The apostrophe makes the name possessive. The last letter of your last name will determine if you add an “-s” or an “-es”. If your last name ends in -s, -z, -ch, -sh, or -x, you add -es to your last name to make it plural.
James's car or James' car? Actually, both ways are correct. If a proper name ends with an s, you can add just the apostrophe or an apostrophe and an s. See the examples below for an illustration of this type of possessive noun.
So a safe solution is to treat singular nouns ending in S the same way you treat singulars nouns not ending in S: Form the possessive with an apostrophe and an S. Thomas's house. The important thing to remember is that Thomas is singular.
The Associated Press Stylebook recommends just an apostrophe: It's Tennessee Williams' best play. But most other authorities endorse 's: Williams's. Williams's means “belonging to Williams.” It is not the plural form of Williams. People's names become plural the way most other words do.
If your last name is Johnson and you want to send a card from your family, simply add an "s": The Johnsons ("Merry Christmas from the Johnsons!"). Only use an apostrophe when you want to make a name possessive. ("From The Smith's" is always wrong, but "The party is at the Smiths' house" is correct.)
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