The tiniest things can be so useful when you come to consider the nuts and bolts of writing. The comma is one of them. No, don't go away: it's a useful device that's often used badly -- or ignored altogether.

As an editor of books, I am forever sticking commas into other people's prose, having first of all had to work out what their sentence is trying to say. Many people are slapdash about comma use, and it can go down badly with a commissioning editor who is, say, looking at your approach letter and sample chapter and thinking, "Uh, oh. This one's going to give us problems. Where's that equally promising but better-presented proposal that will cost us far less in copy-editing fees?"

There are many useful punctuation and grammar tips in a downloadable book I co-wrote on how you can get yourself published very quickly, You Can Write Books (at www.youcanwritebooks.com), although its main focus is on how to get your work before a publisher. But here I'll deal with this one useful but sometimes overlooked little squiggle, and a few things that many writers neglect.

First, should be used in a list of items to break them up: pens, pencils, books, and paper. Should you use a comma before and in that list, though? Well, yes if you're, say, a North American; yes and no if you're British. This is called the list comma or serial comma (it is often called the Oxford comma, and is still used by Oxford University Press).

Many people say it's not necessary, because the word and in that sentence is doing the job the commas were doing earlier in the sentence. However, many writers of English throughout the world -- notably, as I said, in America -- like the serial comma, and insist on it. Most British writers, publishers, and newspapers don't bother with it, and so would write "writers, publishers and newspapers"; "pens, pencils, books and paper."

What should you do? Well, you choose a style and stick to it. Be consistent. Better still, you see which style your potential publisher uses, and go with that. That goes for all stylistic considerations. Don't forget: you're selling an idea to a publishing house, and anything you can do to impress them will go in your favor.

The comma is also very handy to separate two distinct clauses in a sentence, and this is where many writers ignore it. You'll see how I used it in the last sentence, after the word sentence. Not only does it give you a pause for breath (a mental breath if you're not reading aloud, of course), but often it can help the meaning.

Take this: "I passed the ball to Joe and Fred kicked it into the net." I passed the ball to Joe and Fred? No, I passed it to Joe, and then Fred came along and kicked it into the net. You get the meaning eventually, but why should I as writer give you, the reader, pause? If I'd written, "I passed the ball to Joe, and Fred kicked it into the net," you wouldn't even have blinked, because the meaning would have been immediately clear.

Or look at this sentence: "That day I went to the movies ..." could mean that on that particular day I went to the movies ("That day, I went to the movies") or that it was on the day I went to the movies that something else happened ("That day I went to the movies something else happened").

In the first pair of parentheses above, you'll see how the comma has been used to convey the first of the two possible meanings. But, if you miss it out, you're going to have your reader thinking you meant it the other way. If that's not your intention, make sure you use the comma.

A few final brief points, then, about the comma.

Commas come before quoted speech: "Joe said, 'Let's go and see a movie.'"

Commas come after a piece of quoted speech, before you say who's speaking: "'Let's go and see a movie,' said Joe."

Commas can be used in the way brackets are used: "My English teacher, Martha Moonbeam, gave me good marks this week." The commas here are known as bracketing commas, because they do the same work as parentheses (round brackets, like these) but in a "weaker" way.

Another example of bracketing commas would be, "The train, which was late leaving, made up for lost time." Don't miss off the second comma; many people do.

Commas separate adjectives in such sentences as, "It was a cold, damp, dreary sort of day." In this context, they're doing much the same work as in the list of items above, except that here we have adjectives, not nouns.

In one short article, you have, I hope, got to grips with this bit of occasionally troublesome but very useful punctuation. You may have been familiar with some of the things, anyway, but I hope that, if there were one or two concerns you weren't too sure about, this article has been of help.

Andrew John is co-author of You Can Write Books, a no-nonsense downloadable book from http://www.youcanwritebooks.com that will get you into print if you follow its advice. He and his co-author, Stephen Blake, have written more than a dozen print titles (details on http://www.youcanwritebooks.com). Both are writers and editors, and You Can Write Books is crammed with advice you can trust.