There’s this little thing in the grammar world called an Oxford comma. Basically, it is the comma placed after the conjunction in a series of words in a sentence. And yes, it’s totally confusing and the cause of much debate.
Let me try to break it down for you. Here are two sentences, one of which uses the Oxford comma:
a. After it happened, I only talked to my therapists, my mom and my dad.
b. After it happened, I only talked to my therapists, my mom, and my dad.
In the first sentence, ‘I’ is implying that the mom and dad are the therapists, making it seem like only two other people are involved. The second sentence clarifies that the subject is speaking about three or more people.
Neither of these are incorrect, depending on who you talk to. If you ask most grammar enthusiasts, the Oxford comma is accepted and encouraged for clarification. But if you are dealing with someone who works in the news industry and uses the AP Stylebook, the sentence is correct if you omit the last comma entirely. As both of these things, I personally think the Oxford comma is valid in certain instances and I use it when necessary.
Source: Daily Infographic