Plural possessive ending in s

Ed Smith’s house, the Smiths, the Smiths’ house.

But when name ends in s or z:

Ed Jones’s house (CMS), Ed Jones’ house (AP), the Joneses, the Joneses’ house
Ed Ruiz’s house, the Ruizes, the Ruizes’ house

Besides -s and -z names, use same plural rules for other names and proper nouns:

and the Kellys, the Menlos, the Miglinis, etc.
two Jennys, two Germanys, two St.

Louises, two Katzes

But possessive rules diverge a bit:

Kansas’s farms (CMS), Kansas’ farms (AP)
BUT the United States’ farms (because plural word is used as singular: the United States is, not are).

Possessive common nouns:

the dress’s fabric, the dresses’ fabric

BUT if next word starts with s:

CMS treats it normally: the dress’s stitching
But AP says the dress’ stitching because stitching starts with s
Both agree on normal plural: the dresses’ stitching

Most names, add s

the Smiths, the Cases, the Nasrs
the Bradys, the Cavallos, the Morellis

Ending in s or z—add es

(also, j, x,

first names in plural