Sadly, many children have no idea when it comes to using the apostrophe of possession correctly. They will be writing when, suddenly, they presumably think to themselves that they haven’t used one of those ‘commas in the air’ and proceed to use one in a word that bears no connection to ownership.
Even worse, from what I have witnessed on television, quite a few adults do not place such an apostrophe in the correct position, either!
I trust that the following is of help to those who are unsure.
I find that if one inverts the relevant part of a sentence, where to place the apostrophe of possession becomes more obvious.
Example: the babys bottle becomes “the bottle of the baby”
In this instance the apostrophe is placed after the last word in the inversion, which, in this case is ‘baby’. All that remains to be done then, is to add the ess: the baby’s bottle.
Example: the babies bottles becomes “the bottles of the babies”
As ‘babies’ is plural, we only add the apostrophe after the last word ‘babies’, hence, the babies’ bottles.
Example: Smithville Girls High School becomes “Smithville High School (for) Girls”.
Again, the same rule applies and the apostrophe is placed after the last word, namely ‘Girls’.
As ‘Girls’ is plural no ess needs to be added: Smithville Girls’ High School.
Example: Mothers Day becomes “a day for mothers”
Applying the same rule, it becomes Mothers’ Day.
Sometimes a second ess should be sounded and, therefore, added.
Examples: Spartacus’ sword (“the sword of Spartacus”) becomes Spartacus’s sword.
Hughes’ boatshed (“the boatshed of the Hugheses”) becomes Hughes’s boatshed.
Amos’ pride (“the pride of Amos”) becomes Amos’s pride.