Take a close look at this sentence about the great playwright Tennessee Williams: It’s Tennessee William’s best play. Note the placement of the apostrophe. It disfigures the name Williams—how could that be right? Here’s a rule to live by: Forget the apostrophe until you write out the entire word. A correct possessive apostrophe can never entangle itself within any word. So by writing Williams out first, you can avoid a lot of trouble.

The trouble that can’t be avoided comes next, because there are conflicting policies for writing possessive proper nouns that end in s. The Associated Press Stylebook recommends just an apostrophe: It’s Tennessee Williams’ best play. But most other authorities endorse ’s: Williams’s.

Williams’s means “belonging to Williams.” It is not the plural form of Williams. People’s names become plural the way most other words do. Only rank amateurs think the plural of cat is cat’s. Names are no different. They seem different because of human vanity: we’re somehow reluctant to compromise the “purity” of Smith so we mistakenly write the Smith’s, adding the apostrophe to establish a respectful distance between the name and the s rather than simply writing the Smiths, the Fongs, the Calderóns.

Now, what if the name ends in s? Figuring out the plural of a name like Williams drives people crazy. Some would write the Williams, but that means the family’s name is William. Others employ that misguided apostrophe: the Williams’ or the Williams’s or even the William’s. That last one is particularly ghastly. Taken literally, the William’s means something ridiculous: “belonging to the William.” Forcing an apostrophe between the m and s mangles and mocks the name.

All names ending in s become plural by adding es. Make it the Williamses. To show possession, add just an apostrophe: Williamses’. The house belonging to the Williams family is the Williamses’ house. Maybe you’re thinking it sounds ridiculous and looks bizarre. But it’s also correct.

Let’s look at some other types of proper nouns …

• Many organizations, companies, and government agencies are known by two or more capital letters (AP, MGM, EEOC). Initialisms ending in S show possession by adding ’s: CBS’s ratings, DHHS’s policies.

• Most writers and editors make an exception for biblical and classical proper names ending in s. Traditionally, only an apostrophe is added to such names: Moses’ law, Xerxes’ army. However, the influential Chicago Manual of Style recently ruled against this odd policy and started recommending Moses’s, Xerxes’s, etc.

For apostrophes with possessive proper nouns, remember these three guidelines: If the noun is singular, add ’s (Kansas’s). If the noun is plural but does not end in s, add ’s (the Magi’s gifts). If the noun is plural and ends in s, add just an apostrophe (the Beatles’ greatest hits).

Except for writers who abide by Associated Press guidelines, apostrophe rules for possessive proper nouns are virtually identical to those for possessive common nouns.