date / year                  April 9,2001

            numbers                     123,456

• to separate elements that are compounded or in a longer series
• to bracket off introductory sentence modifiers
• to bracket off inserts
• to avoid ambiguity

The boy,his sister,and his dog went swimming.

verbs

           They ran,swam,and laughed.

full predicates

           I  moved here,found a job,and sent my kids to school.

and even when joining sentences when using a connecting word

This is where I call home,sothis is where I'll die.   

Finally, we can use commas to separate a series of noun phrase premodifiers

            The big,bad,boisterous baboon......

In all of the cases above, the comma serves asand.A comma occurs after all but the next to last item in the series.

Commas are thus used to separate elements that are compounded or in a longer series

Because he loved her,he did not marry her.

The final use commas is to avoid a potential misunderstanding, such as when a sentence modifier at the end of a sentence may be mistaken for part of an earlier chunk, as with our sentence

He did not marry her, because he loved her..

And that's it. Any other commas (such as between subject and predicate), does not belong.

Try this experiment: Give your instructor five dollars for each comma you use in an essay. Your instructor will return five dollars for each comma used correctly. You should come out even. This technique for cutting down on unwanted commas has been heartily endorsed by every English instructor who has tried it.

Rules for Comma Usage, http://webster.commnet.edu/hp/pages/darling/grammar/commas.htm

The relationship of structure and meaning is clearest in the case of ambiguity. Equally legitimate analyses yield equally legitimate (however unintended) meanings. Recall that sentence modifiers and inserts can both appear at the beginning or end of a sentence. This produces a situation with significant possibilities for ambiguity. How then are we to decide if an element at the beginning or end of a sentence is

• a sentence modifier
• an insert
• or even part of the simple sentence itself,

and with that which meaning of the sentence to accept?

We make such judgments on the basis of the nature of the content and the earlier tests.

·        if the element helps shape the overall meaning and can shift from front to back and vice versa, it's probably a sentence modifier.

·        if removing the element produces an incomplete or meaningless sentence, the

construction is part of the simple sentence.

·        if removing the element results in the loss of additional information or an editorial comment, but does not change the basic sense of the sentence, the construction was probably an insert.

Life gets interesting when two or more of these situations exist at the same time.

We read with attention to both the content and the structure of the sentences, to both the thought expressed and the grammatical structure. Each informs the other.

Unlike the New Zealand Expeditionary forces, which gave condoms to their soldiers, the United States decided to give American soldiers after-the-fact, and largely ineffective, chemical prophylaxis.

To make sense of this sentence, we must recognize three basic elements. There is the core sentence:

Unlike the New Zealand Expeditionary forces,

  which gave condoms to their soldiers,

the United States decided to give American soldiers after-the-fact,

and largely ineffective,

chemical prophylaxis.  

There is the sentence premodifier indicating that the action was different than that taken by the New Zealand Expeditionary forces.

Unlike the New Zealand Expeditionary forces,

  which gave condoms to their soldiers,

the United States decided to give American soldiers after-the-fact,

and largely ineffective,

chemical prophylaxis.  

And there are two inserts that supply the information that the New Zealand Expeditionary forces gave their troops condoms and that the after-the-fact American efforts were largely ineffective:

Unlike the New Zealand Expeditionary forces,

  which gave condoms to their soldiers,

the United States decided to give American soldiers after-the-fact,

and largely ineffective,

chemical prophylaxis.  

One way or another, we have to find structure in the sentence to make sense of it.