7. Possessive Case

A) Adding an apostrophe to words ending in "s".

One method, common in newspapers and magazines, is to add an apostrophe plus s (’s) to COMMON nouns ending in s, but only a stand-alone apostrophe to PROPER nouns ending in s.

Examples:

• the class’s hours
• Mr. Jones’ golf clubs
• The canvas’s size
• Texas’ weather

Another method is to write the word as we would speak it. For example, since most people saying “Mr. Hastings’ pen” would not pronounce an added s, we would write Mr. Hastings’ pen with no added s. But most people would pronounce an added s in “Jones’s,” so we’d write it as we say it: Mr. Jones’s golf clubs.

B Plural possessives

To show plural possession of a word ending in an s or s sound, form the plural first; then immediately use the apostrophe.

Examples:

• the classes’ hours
• the Joneses’ car
• guys’ night out
• two actresses’ roles

Further examples are available here