There's a tendency these days to treat the last comma in a series as optional. It has spread itself like a true grammatical plague, infecting even published works! A series is any listing such as

apples, oranges, and bananas

or any set of more complex ideas such as

filtering out daylight, applying epoxy to the surface, and praying for success.

Many writers and editors take a lax view of the final comma, even seeming to discourage its use. Is it worthwhile to save those pennies on printers' ink by omitting the ultimate comma? As we shall hopefully see, the answer is no. Let us explore the reasons for the last comma in a series.

1) Demarcation

When a writer chooses to include the final comma for a series, the reader is assured that he or she knows exactly when any such series will reach an end: when it is punctuated by ", and" or another conjunction.

a) Ex:

Unclear:
Thin as a rail, bait and switch and pound for pound are all clichés.

Better:
Thin as a rail, bait and switch, and pound for pound are all clichés.

2) Clarity

The proper understanding of a sentence often hinges on the placement of a word or a mark of punctuation. Writers that forgo the final comma in a series can introduce unintended ambiguity, running a serious risk of allowing their readers to misunderstand their meaning.

a) Ex:

Ambiguous:
Quinn: "Give me some credit, straighten up and think about what you did."

Better:
Quinn: "Give me some credit, straighten up, and think about what you did."

In the "Ambiguous" example above, it sounds as though Quinn is giving two distinct directives: the first to give her some credit, the other to straighten up and think about what was done. Such a construction would indicate a comma splice, which is an incorrect joining of two independent clauses with a comma. We may assume the writer made such a mistake. It isn't immediately clear that all three directives are given together as part of a single series of directives, as we can see easily in the "Better" example.

b) Ex:

Ambiguous:
There are two ways to stop a bear, throw a knife and climb a tree.

Better:
There are two ways to stop a bear, throw a knife, and climb a tree.

Again, we face an unclear construction with the "Ambiguous" example above. Because of the wording, we may be tempted to read the throwing of a knife and the climbing of a tree as examples of the two ways to stop a bear, seeing perhaps that the writer is using the comma incorrectly (instead of the appropriate colon if our initial read was the intention of the author). But when we read the "Better" example, we see that writer really intended to list a series of activities that are related by the fact that there are two ways of doing each of them! Now if a survival guide were written without that final comma we'd be in some trouble, as neither throwing a knife nor climbing a tree will subdue the threat of a bear attack! (Contrary to popular belief, most bears can climb trees.)

3) Consistency

Finally, since I've shown how the lack of the ultimate comma in a series can lead to confusion and misreadings, we should therefore always use the final comma to both avoid problems and maintain consistency. Consistency is one of the most important aspects of a properly written work. Even writing riddled with mistakes can be readable if the mistakes are consistent! So, because the context will often require the last comma in order to prevent a mistake, we may as well always use it to provide both correctness and consistency. At the least, including it is never wrong.

How can it be that the lack of the final comma in a series is such a pervasive approach, even common in serious published work? I do not have the answer to such a question. My guess is that lazy or lackadaisical attitudes are to blame. Seeing that omitting the comma can be done in many situations without error, writers and editors set it as their default. One less mark, one less stroke of the key, one less thought. Perhaps even minute printing costs were assessed. In any case, the pains must outweigh the benefits, as a more disciplined approach to writing is in benefit of the reader, the writer, and the written word as a whole. Just remember next time you see such an omission (and you will) that you and I know better.