"Special problems arise when you create possessives for names already ending in s. Is it Charles’ Wain or Charles’s Wain? The latter sounds and looks better. Is it St James’s Street or St James’ Street? Custom and rhythm combine in urging the former. Jones’s house indicates that only one person named Jones lives there; if a family does, it should be the Joneses’ house, which sounds the same but looks odd on the page. Until recently, the usual form was Jesus’, not Jesus’s, but this tradition, which has been described in Hart’s Rules as “an acceptable liturgical archaism”, was finally broken in the New English Bible of the mid-sixties."
"Many people struggle with the possessive case of singular nouns when the words already end with s. The general rule is this: Form the possessive singular of nouns with 's.
Here are some examples:
* James's cat * Mrs. Jones's attorney * Dr. Seuss's book"