Styles vary, with some styles adding apostrophe S (Thomas's), especially because the new S is sounded (sounds like the plural Thomases).
Others, notably the AP Style Guide, use only an apostrophe (Thomas') but still add the pronunciation as another S sound.
The Thomases is the plural form of the proper noun Thomas.Adding an apostrophe (Thomas's and Thomas') forms the singular possessive noun. Both forms are the accepted possessive form.Examples:The Thomases have a beautiful garden. (plural)Dr. Thomas' office is on the second floor. (singular possessive)Dr. Thomas's office is on the second floor. (singular possessive)The Thomases' garden is beautiful. (plural possessive)
The singular possessive form is week's.
The singular possessive form is classmate's.
The possessive form is Amos's.
The singular possessive form for the noun gypsy is gypsy's.
The singular possessive form for country is county's.
The singular form for the plural noun countries is country; the singular possessive form is country's.
singular possessive: boy'splural possessive: boys'
The singular possessive form is speculum's.
The singular possessive form is aircraft's.
The singular possessive form is frontiersman's.
The singular possessive form is contralto's.