"The Duke his decree" meant "The Duke's decree."

The apostrophe now stands for those left-out possessive pronouns. The simplest rule for an apostrophe: indicate the possessive by adding 's to all singular nouns and plural nouns that don't end with "s":

Amherst's glee club

the women's soccer team

Add an apostrophe to plural nouns ending in s:

the serpents' teeth

the Furies' wings

Williams' Glee Club 

(Williams's Glee Club is also proper, but never William's Glee Club unless you're talking about a singing group belonging to Prince William.)

Possessive pronouns (whose, its, your, hers, his) NEVER take the apostrophe. Note it's = it is, its = possessive. (Memorize this sentence!)

Whose is that umbrella? It's hers, but its handle is broken.

Apostrophes also indicate letters left out in contractions:

'Twas brillig, and the slithy toves.... ('Twas = it was)

Who's going to claim the umbrella whose handle is broken? (who's = who is)

It's a long way to Tepperary.... (it's = it is)

Use a contraction with a proper name when indicating a word left out:

We're going to Hahjee's for dinner. (Hahjee's stands for Hahjee's Place and is a possessive.)

But NEVER use an apostrophe with a plain old plural noun, in the manner of unlettered hand-lettered signs:

(WRONG) Wild Animal's From Foreign Countrie's

(WRONG) Lobster's, Crab's, and Mussel's for Sale

   Sentence Fragments
   Comma Splices
   Run-on Sentences
   Dangling or Misplaced Modifiers
   Subject-Verb Agreement
   Split Infinitives
   Which, That, It, and Of
   Semi-colons and Colons
   Apostrophes
   His and Her