To form the possessive of a singular noun not ending in an s sound, add an apostrophe plus s to the noun. (But you knew all these…)
- my lawyer’s advice
- my child’s teacher
- Tulabelle’s haircut
- Mr. and Mrs. Snerd’s woodpile
- Illinois’s politicians
- Arkansas’s former governor
- Des Moines’s mayor
- the corps’s leadership
To form the possessive of a singular noun that ends in an s sound, be guided by the way you pronounce the word.
If a new syllable is formed in the pronunciation of the possessive, add an apostrophe plus s.
- your boss’s approval
- the witness’s reply
- Ms. Lopez’s application
- Mr. and Mrs. Morris’s plane tickets
- St. Louis’s airport
- Dallas’s football team
- Congress’s failures
If the addition of an extra syllable would make a word ending in an s hard to pronounce, add the apostrophe only.
- Officer Phillips’ water ski
- Judge Hastings’ decision
- the Burroughs’ condominium
- Los Angeles’ freeways
- New Orleans’ restaurants
- Jesus’ parables
- Moses’ flight from Egypt (it was ‘grounded’ but arrived nevertheless)
- for goodness’ sake
- Achilles’ heel [but: Achilles tendon]
NOTE: Individual differences in pronunciation will affect the way some of these possessives are written. For example, if you pronounce the possessive form of Perkins as two syllables, you will write Mr. Perkins’ kindness; if you pronounce the possessive of Perkins as three syllables, you will write Mr. Perkins’s kindness. The important thing is to listen to your own pronunciation. When you hear yourself pronounce the possessive of boss as two syllables (boss’s) and the possessive of witness as three (witness’s), you will not be tempted to write your boss’ approval or the witness’ reply. Naturally, tradition should take precedence over your ear. For example, the U.S. ambassador to Great Britain is appointed to the Court of St. James’s (not, as you might expect, Court of St. James).
When forming the possessive of any noun ending in s (for example, Mr. Hodges), always place the apostrophe at the end of the original word, never within it.
- Mr. Hodges’ message (not: Mr. Hodge’s message)
Source: The Gregg Reference Manual.