What are the rule of possessive form of plural noun?

The rules for plural possessive nouns are:

• If the plural form already ends with -s, add an apostrophe after the existing -s.
  • teacher, teachers, teachers'
  • street, streets, streets'
  • lady, ladies, ladies'

• If the plural form does not end with -s, add an -'s to the end of the Word.
  • child, children, children's
  • sheep, sheep, sheep's
  • man, men, men's