Possessives

According to The Chicago Manual of Style (which many people regard as the editors' Bible),

1 Singular Nouns	noun	possessive
common noun [singular]	dog	dog's The dog's bowl is empty
proper noun [singular]	Gerry	Gerry's Gerry's bowl has teeth marks on it.

2 Plural Nouns	noun	possessive
common noun [plural]	dogs	dogs' The dogs' bowl(s) are empty
proper noun [plural]	Toms	Toms' The two Toms' trials began when their Suburban got a flat.

3 Singular Nouns (w/ "s" ending)	noun	possessive
common noun ending in "s" [singular]	bus	bus's The bus's route went by Newt's house.
proper noun ending in "s" [singular]	Lewis	Lewis's John Lewis's vibraphone is missing. [ Note exception 1]

4 Plural Nouns (w/ "s" ending)	noun	possessive
common noun ending in "s" [plural]	buses	buses' The buses' fumes spread across the intersection.
proper noun ending in "s" [plural]	Lewises	Lewises' The Lewises' home was burglarized during their trip. [Note exception 2]

Exception 1: According to The Chicago Manual of Style, when a proper noun ends in "s" and is 3 or more syllables [Ex. Sophocles], then the possessive is made by simply adding the apostrophe after the "s." For the previous example, that would be Sophocles' play.

Also, if a proper noun has more than one "s" in it [Ex. Jesus, Moses], then the possessive is made by simply adding the apostrophe after the "s." For the previous example, that would be Moses' disappointment.

Exception 2: Again, according to The Chicago Manual of Style, when a word is of foreign origin and ends in a silent "s" [Ex. Beaujolais, Illinois], then there is no difference in how the singular [Beaujolais'] and the plural [Beaujolais'] is written.