The placement or omission of the final comma in a list may introduce misunderstanding or dispel it; it may change the meaning.
Peter left for Sweden with his parents, Ann and Jason.
The question rightfully arises: How many people went to Sweden?
Peter left for Sweden with his parents, Ann, and Jason.
With the use of the serial comma, we know that five people went to Sweden.
Conclusion: If you want to say five people left and you do not want confusion, use the final comma.
Now let us take a look at another sentence:
What I like is sour apple, sour cream and cherry.
There is a chance to interpret this sentence thus:
sour apple, sour cream and sour cherry.
By placing the comma, you deprive the sentence of that possible reading.
Bottom line: Use the final comma when the sentence so requires.
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