the noun referred to, that is, the plural, though for the sake of smoothness and euphony, omit the succeeding (or rather non-succeeding) "s." Thus, "the boss's desk" in the singular, "the bosses’ desks," in the plural. When the singular ends in "s," the possessive "s" is usually retained, excepting where the noun has three or more syllables and the word following commences with this letter. Thus, Charles's uncle; Burns's poems; Burns's stanza; Damocles' sword. The possessive "s" is also generally omitted before "sake"—as, "For conscience’ sake' (conscience having the "s" sound); "for Jesus' sake."
In speaking of a firm, where the partners constitute but one object of contemplation, the apostrophe is used but once—after the complete object of contemplation, that is, after the title or firm name; as, "Jones and Robinson's store." If Jones and Robinson, instead of being in partnership had independent businesses you would speak of "Jones's and Robinson's stores"—this being no exception to, but merely an exemplification of, the rule that the apostrophe immediately follows the noun or name (or firm name) under consideration.
Occasionally, the possessive appears in double form, the substantive being preceded by of and followed by the apostrophe with s. This occurs, however, only in idiomatic phrases,as, "He was a friend of my father's," which is equivalent to "He was one of my father's