Comma Rule 6: Commas that express contrast 

Using commas is crucial to contrast between to different things. 

He is small, but fast.
The car is red, but old. 

Sometimes, the contrast is embedded in a longer sentence. 


It's not about the cover, but the content, that makes a good book.
This could be a par of the first comma rule but is isn't


You don't have to use a comma when emphasizing the contrast.


He is small and fast.
The car is read and old.


Link:
http://thecraftedword.weebly.com/comma-rule-6.html