When you want to show that something belongs to someone, adding an apostrophe and an s is usually simple. However, the rules get more complex when two people own something together, when a noun is already plural, or when you are talking about time.

Joint vs. Separate Possession

When two or more people own something together (joint possession), you only add the apostrophe and s to the last name in the list. The single apostrophe shows that the ownership is shared.

However, if two people each own separate things (separate possession), you must add an apostrophe and s to every name to show that the items belong to them individually.

✅ Arthur and Merlin's shared apartment is on the third floor. (They share one apartment). ❌ Arthur's and Merlin's shared apartment is on the third floor.

✅ Mark's and Sarah's individual suitcases are by the door. (Mark has his suitcase, and Sarah has hers). ❌ Mark and Sarah's individual suitcases are by the door.

Irregular Plural Possessives

For regular plural nouns ending in s (like employees or babies), you simply add an apostrophe at the very end of the word to show possession: the employees' breakroom, the babies' toys.

But English has many irregular plural nouns that do not end in s—such as women, children, men, and mice. Because these words don't already end in an s, you must treat them like singular nouns and add 's.

✅ The children's educational toys are on the second floor. ❌ The childrens' educational toys are on the second floor.

✅ The mice's tiny footprints led straight to the cheddar vault. ❌ The mices' tiny footprints led straight to the cheddar vault.

Time Expressions and Compound Nouns

Apostrophes aren't just for people and animals; they are also used in time expressions to show duration or measurement. This acts as a replacement for the word "of" (e.g., a delay of three months becomes three months' delay). If the time period is singular, use 's. If the time period is plural, use s'.

✅ The astronaut reported a three months' delay. ❌ The astronaut reported a three month's delay. ✅ I will be ready in one hour's time.

For compound nouns (words made up of two or more words joined by hyphens), the rule is straightforward: always attach the 's to the very last word, regardless of which word is pluralized.

✅ The commander-in-chief's horse refused to move. ❌ The commander's-in-chief horse refused to move.

Common Mistakes: Whose vs. Who's and Nouns Ending in S

A frequent mistake is confusing the possessive pronoun whose with the contraction who's (which means who is or who has). Remember that possessive pronouns (yours, hers, ours, whose) never take apostrophes.

✅ I need to know whose half-eaten donut was left on the machine. ❌ I need to know who's half-eaten donut was left on the machine.

Finally, learners often freeze when a singular noun ends in s (like boss, princess, or bus). The most standard and widely accepted rule in modern English is to add 's, just as you would for any other singular noun. This matches how we pronounce the word, adding an extra "iz" sound at the end.

✅ The boss's new policy was highly unpopular. ❌ The bosses new policy was highly unpopular. ✅ The princess's missing tiara caused quite a scandal.