Singular possessive nouns
When you want to indicate to the reader that the thing described by one noun owns something else you can turn it into a possessive noun. This is usually achieved by adding an apostrophe and the letter ‘s’ to the end of the noun. Take this simple sentence for example:
I saw Bob’s dog the other day.
Normally, the name Bob is a proper noun. But in this sentence we want to talk about the dog that is owned by Bob. We can do this by adding an apostrophe and an ‘s’ to the end of ‘Bob’. This is an example of a singular possessive noun - there is only one Bob who owns the dog.
When the noun that you’re trying to change into a possessive noun already ends with an ‘s’, you can do one of two things. You can just add an apostrophe, or you can add an apostrophe and another ‘s’. If the noun we’re trying to change is ‘class’, here’s the two ways:
The class’ average mark was very good.
The class’s average mark was very good.
Often, one way will look a lot better than the other way, although you won’t always be able to say exactly why this is so - just go with the one which you think looks better. One thing that will help you is remembering that if a word ends in a silent ‘s’ or an ‘x’, add both an apostrophe and an ‘s’ to make it possessive. For instance, the word ‘bass’, which is pronounced ‘base’ (at least when you’re talking about music), ends with an ‘e’ sound, not an ‘s’ sound - the last ‘s’ is silent. So add an apostrophe and an ‘s’ to make it possessive:
The double bass’s sound is very deep and soothing.