Comma Usage in a List

Almost 20 years ago, I was sitting in grade school learning about the comma when my teacher explained to me how a comma is used to iterate a series of items. However, she then explained that a comma was optional to separate the last two items of the list. I objected, saying there had to be a right way. I told her that a comma should separate the last two items in the list since it separates all the other items in the list. But she said it wasn't necessary and I didn't understand.

Well, I can acknowledge today that I finally understand. After living my life in complete support of full usage of the serial comma, I see the potential ambiguity of it (see Wikipedia reference below). There are cases where having the comma separate the last two items can lead to a misunderstanding. So I was wrong in my assertion in grade school about always putting a comma to separate the last two items in a list.

However, I do believe that there always is a correct usage of a comma for a given sentence (or a reasonable alternative to using a comma). So one cannot say that it is "optional" to use a comma to separate the last two items in a list, even if it leads to ambiguity. For a given sentence with a list, there always is a way to form it to allow a read full understanding.

I will sleep better tonight knowing this information.

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From Wikipedia:

"Commas are used to separate items in lists, as in They own a cat, a dog, two rabbits, and six mice. In English a comma may or may not be used before the final conjunction (and,or, nor) in a list of more than two elements. A comma used in such a position is called a serial comma or an Oxford or Harvard comma (after the Oxford University Press and Harvard University Press, both prominent advocates of this style). In some cases use or omission of such a comma may serve to avoid ambiguity:

Use of serial comma disambiguating:

• I spoke to the boys, Sam and Tom. - could be either the boys and Sam and Tom or the boys, who are Sam and Tom (I spoke to two people).
• I spoke to the boys, Sam, and Tom. - must be the boys and Sam and Tom.

Omission of serial comma disambiguating:

• I thank my mother, Anne Smith and Thomas. - The writer is thanking three people: the writer's mother, Anne Smith (who is not the writer's mother) and Thomas.
• I thank my mother, Anne Smith, and Thomas. - Could be either my mother and Anne Smith and Thomas or Thomas and my mother, who is Anne Smith.

If the individual items of a list are long, complex, affixed with description or themselves contain commas, semicolons may be preferred as separators, and sometimes the list may be introduced with a colon."