In civics, a banner is spangled with stars. At the ballpark, it's a bedsheet with a cheer spray-painted on it. On the Net, a banner is a commercial, urging you to click it, PLEASE, and be whisked off to a place of ambush, where well-oiled cyberhucksters will attempt to separate you from your cash.

No one logs onto the Net in order to see these banners. But they are as much a part of the online experience as zebra mussels are to sailing.

And they have their own art. I saw a banner exchange ad today that asked the rhetorical question, ARE YOU AN IDIOT? YES/NO. I had to restrain myself from clicking the NO. (NO does the same thing as YES, which tells you all you need to know about idiots.)

What is a banner exchange? A banner exchange displays your banner ad on other websites, at no cost to you, if you agree to show other people's ads on your site, on a rotating basis -- everyone visiting you sees a different ad. It's easy to do.

There's a long list of banner exchange services, including leaders like LinkExchange, along with the me-toos and wannabes, on Yahoo at this site.

If you are going crazy trying to lure people to your site, a banner exchange sounds like a good deal: free exposure. But there are, as there so often are, catches. v

For one thing, the banner you design, using a graphics program or a generic ad, has to work. Think about how often you have clicked on a banner, and why. A successful banner must communicate a believable benefit, yet it must be small in dimensions and in kilobytes. v

You compromise your web page when you put other people's ads on it. Especially since the exchanges urge you to post it front and center, so it's the first thing your visitors see. If the ad is more attractive than your site, people may click it and be on their way, spending all of three seconds admiring your handiwork. v

Their ad may make your skin crawl. Exchanges allow you to refuse ads for outright X-rated sites, but that leaves lots of room for dubious stuff like CINDY CRAWFORD WANTS TO MEET YOU. v

Even family-safe banners are usually visual pollution, like billboards on a coastal highway. And slow: a 4-kilobyte banner uploaded from around the world may take as long to load as your entire 100-kilobyte site. v

Simply drawing traffic to your site shouldn't be your goal. Drawing appropriate traffic, that may be interested in your topic, should be your goal.

Remember, too, that banner exchanges are in business for a reason, and it isn't to help you. They get the use of your site at no cost to them. Their real business isn't "exchanging" at all, but selling paid advertisement to businesses who want to buy their way onto the Net. Their talk about credits and click-ratios makes you giddy, but the value is right up there with Confederate money.

Surfing through the net the other day, I was surprised to discover that a banner I created enjoyed a top "clickthrough" ratio, at a place called 1-2-Free. (Now out of business - 9/21/98.) I recall creating it, 18 months ago, as a joke. It is a cartoon picture of a flame with a nasty little grinning face, and the legend: HOT ENOUGH FOR YOU?

This ambiguous, could-mean-anything splotch was enjoying a robust 17% click-rate, meaning that 17% of people seeing it just had to find out what it meant. All it meant was, visit my site, please.

I made about a dozen banners in one day. HOT ENOUGH FOR YOU was the best attention-getter, but doubtless left many visitors visiting my mild-mannered site feeling miffed. Others banners were less successful. My banner with a clip-art hex-nut, with the legend BRASS HEX, was clicked once every 68 times it was seen. My clip-art housefly inviting visitors to BE A FLY ON THE WALL did even worse, achieving a click rate 1:128.

But my banner urging people to LOSE BIG MONEY! did worst of all, tallying a gruesome click-ratio of 1:534. I have to wonder who that one person was.

Are banners here to stay? They have their place. Banners work satisfactorily for broad consumer categories like cars, discount airfares and CDs.

But like e-mail spam, they are too "broadcast" to be respectable. Honorable businesses will avoid banners because of their indiscriminate approach, and their association with porn.

Smart marketers are experimenting with Java scripts, pop-up windows, and push methodologies that target individual user needs, and give them greater control and precision.

Trying to stop web traffic with a banner is like trying to stop actual traffic by standing in the street and waving your arms. If you're the driver, you'll be too annoyed to stop. And if you're the person waving, do you really want to do business with whatever slackjawed weirdo slows down for you?