Everyone remembers this rule: the comma in a series. Let’s look at an example.
The buffet features trays of fish, vegetables and crackers and cheese.
We agree there is a tray of fish. But then things become murky. The second tray could contain vegetables and crackers while the cheese flies solo. However, it would also make sense to serve the crackers and cheese on one tray and to reserve a tray for vegetables. One glimpse onto the buffet table can clear this up, but so can the Oxford comma.
The comma after the second to last item in a list of three or more is called the Oxford comma. It is a bit old school, often unnecessary and, therefore, often shunned in the interest of economy of space. However, there are instances where the Oxford comma is indispensible. In the interest of clarity and consistency, I am hanging on to the Oxford comma in my own writing and edit accordingly.
In case you were wondering:
The buffet features trays of fish, vegetables, and crackers and cheese.