I have been doing quite a bit of writing and, for the life of me, cannot figure out why sometimes you have a comma before the “and” then sometimes you do not.
So I asked world-class editor Anja Pujic (@languageadvice) who explained it this way:
About the comma…
Items in a series are normally separated by commas. In book publishing, the and before the last item in the list is always preceded by a comma. This comma is referred to as the serial comma.
He bought apples, bananas, and strawberries.
Example 2 (more complex):
She put on her shoes, tied them up, and ran out the door.
Another place you’re likely to see a comma before and is when it joins two independent clauses.
He let out a loud cry, and everyone turned to look at him.
John opened the door and left. (no comma before and)
In the first example, “He let out a loud cry” is one independent clause, and “Everyone turned to look at him” is the other independent clause. They’re joined by the coordinating conjunction and. The rule is to always use a comma before the and that joins two independent clauses.
Another way of looking at this rule is to say: if both clauses have a subject, then use a comma. In the second example, there is no comma before and because the verbs (opened and left) have the same subject (John).
I hope that’s clear.
I wrote back to Anja saying–
Can you please explain why:
He bought apples, bananas, and strawberries
has a comma before the and while
John opened the door and left.
Anja then said:
He bought apples, bananas, and strawberries. –> This is a list/series, so the and before the final item in the list requires a serial comma to be placed immediately before it.
John opened the door and left. –> This is a compound predicate (two verbs with the same subject), so no comma is required before and. However, if you were to write…
John opened the door, and he left.
…then you would need the comma before and because now you have two independent clauses joined by a coordinating conjunction (and).
Do you see the difference?