For nouns plural in form but singular in meaning, add only an apostrophe:

  • mathematics’ rules, measles’ effects, United States’ wealth

For singular nouns ending in s sounds (but not in s itself), add apostrophe and s:

  • Butz’s policies, the fox’s den, Marx’s theories, Xerox’s product

For names ending with an unpronounced s, add apostrophe and s:

  • Descartes’s Meditations, according to the author, was meant to defend the Christian faith.

For names ending with an “eez” sound, add apostrophe and s:

  • Xerxes’s reign was from 486 to 465 BC.
  • Euripides’s plays seem modern by comparison with those of his contemporaries.

Note: When these forms are spoken, the additional s is generally not pronounced.

Never use an apostrophe to denote the plural of a personal name:

  • the Smiths
  • NOT the Smith’s

To designate possession in a last name ending in s, such as Johns, add an apostrophe with an s

  • Johns’s

Don't use an apostrophe with plural abbreviations of degrees or tests, or with dates:

  • MBAs, SATs, GPAs
  • 1990s, 1860s

> See: plurals