So often, I see a number of people omitting the last comma before a conjunction, because somebody somewhere changed the rules one day and said it was okay to omit it. The argument for omitting the final comma is that the word, “and,” is supposed to suffice for the omission, but as I note in examples posted below, that supposition doesn’t always work.
Most of the time, readers can easily understand sentences written without the added comma, but sometimes the sentences make no sense – or they are hilarious – so in a comedic sense, the omission of that last comma could, if intended, be humorous.
Some punctuation rules have to change to accommodate changing times. The comma usage rule, however, is not the same type of rule as the omission of the second space after a period. Arguments continue over whether or not to press the space bar once or twice after typing a period.
In the days before computers, when everyone used typewriters, the rule was to press the space bar twice. The reason was because it allowed for easy reading; the reader would temporarily pause before heading on to the next sentence.
When computers were invented (YEARS ago) inventors factored in a reasonable space between sentences, allowing typists to press the space bar only once.
So let’s end the argument right now about whether or not to press the space bar once or twice after a sentence. The answer is – if you still use a typewriter, press the space bar twice; if you use a computer, press the space bar only once (caveat – some fonts and font styles, including italics, cause letters to space unevenly, and in those cases, adding an extra space after a period might look better).
Now on to the question of whether or not to place one last comma in a series of three or more words, phrases, or clauses. I found a great example in the following sentence, showing why we need to use commas appropriately (found on EnglishPlus.com in the Grammar Section on Commas):
Incorrect: The street was filled with angry protestors, shouting spectators and police.
(Leaving out the last comma makes it look like the police were shouting, too.)
Correct: The street was filled with angry protestors, shouting spectators, and police.
(Makes it clearer.)
I found another more humorous example on Stack Exchange (also located on wikipedia):
Sentence: To my parents, Ayn Rand and God.
Correction: To my mother, Ayn Rand, and God.
If you wrote the first sentence, readers would deduce that God was one of your parents (yes, I’m fully aware of the argument that God is the father). By changing the word, “parents,” to the word, “mother,” your sentence makes even more sense.
However, separating three items with commas may not always provide enough clarification. From Stack Exchange, we find the following sentence:
He currently lives with his wife, a ferret, and a cat who thinks she is a ferret.
The implication is that “his wife” is a ferret. Also, as one commenter in the thread beneath the article noted, the word, “currently,” causes the reader to wonder if his wife is only temporary. So in addition to placing commas correctly, we need to also look at the structure of our sentences. Do our readers understand what we’re trying to convey?
Ambiguous writing jeopardizes our credibility as a writer. Some people argue that if the sentence or phrase can be clearly understood with or without the comma, we can choose to leave it out. But why force our readers to work so hard at understanding our work? If our goal in writing our blogs, articles, or books is to express ourselves eloquently, the best way to help our audience understand our words is by using them correctly. We need to follow the rules of grammar or learn how to restructure our sentences so they will make sense to our readers.
In addition to writing coherently, I’m also a believer in continuity in writing. If a writer posts a series of three words in one sentence and leaves out the comma, but then several paragraphs later leaves in the comma, readers may note the inconsistency. The saying, “when in doubt, leave it out,” doesn’t, in my mind, apply to commas.
For more on English and grammar rules, please visit GrammarBook.com.