The rule with the possessive apostrophe is this:

If the word is singular, add 's.
If the word is plural and the plural ends in s, simply add an apostrophe.
If the word is plural and the plural does not end in s, add 's.
The following are all correct:

The man's hat. (The hat of the man - singular, so add 's)
The ladies' books. (The books of the ladies - plural ending in s, so add an apostrophe)
The women's shoes. (The shoes of the women - plural not ending in s, so add 's)

A children's book means A book for children. Children is a plural not ending in s, so you must add 's.

mah927 wrote:Could someone please give me an easy to remember explanation of when to use an 's or an s' Thanks in advance.

Moderator: Apologies in advance if I have this in the wrong location.

The rule with the possessive apostrophe is this:

If the word is singular, add 's.
If the word is plural and the plural ends in s, simply add an apostrophe.
If the word is plural and the plural does not end in s, add 's.
The following are all correct:

The man's hat. (The hat of the man - singular, so add 's)
The ladies' books. (The books of the ladies - plural ending in s, so add an apostrophe)
The women's shoes. (The shoes of the women - plural not ending in s, so add 's)

A children's book means A book for children. Children is a plural not ending in s, so you must add 's.

THanks BJP! that goes in to my flashcard list.