A BYPRODUCT OF the tennis explosion is the reappearance of ancient dueling codes under the guise of challenge matches. Every club, every neighborhood with as many as two public courts, every professional organization concerned with morale, seems to have its won version of a ladder or pyramid tournament. Around these structures has grown an elaborate set of customs with echoes of chivalry, the Old West and German universities; a code of honor designed to provide an orderly hierarchy of excellence in a rigid structure.

A player may only challenge upwardly and only a limited number of places. The player challenged, as the insulted party, has the choice of weapons - that is, he picks the time and place of the match. If he declines to accept the challenge within a reasonsable time, he is himself dishonored and dropped behind the challenger in ranking.

As with gunfighters and knights-errant of old, the vagaries of the system have produced a body of legend and folklore just waiting for a Malory or Peckinpah, a Tennyson or Altman to mine. A few local examples will give the kies.

In Rockville, a woman named Roxanne who was at the bottom of her ladder got lucky one day in May a year ago. She challenged two places up and won by default because the other woman was going away for a week. Then she found, quite by accident in the beauty parlor, that she could move up another two places because another player would be in the hospital for at least a week.

She became obsessed with the idea of moving to the top of this very long ladder, even though she could't beat any of the 50 women above her. But she dared not rely only on casual gossip. So she hired a private detective to chart summer vacations, very discreetly but very carefully.

Then she plotted her climb, leapfrogging all available players by challenging others, always on the eve of their departures. One woman was tempted to delay her trip to Martha's Vineyard but couldn't get a later reservation on the ferry.

By August, Roxy was No. 1 on the ladder, top-seeded in the Labor Day tournament. Unfortunately, she was out of shape from not playing and was beaten, 6-0, 6-0, in the first round by number 43, who had played under wraps all season in order to get the lucky draw on Labor Day.

Then there was Arlingon Andy, despised by everybody on his little neighborhood ladder. They hated his fluky spins and slices and joked about his best shots being his foremouth and backmouth and how he could resort to an overmouth in desperation. His favorite ploy was to hit a short lob and yell "Oh, no!" (at himself) just as his oppnent went up for a smash.

Now, one rule of the code is that you can't accept a challenge when you have a challenge pending. Somehow, whenever Andy got to issue a challenge, the other players always had a previously scheduled match. Usually, though, the people he called were out and they never returned his calls.

Undaunted, Andy made use of yet another rule - that a challenge match cannot be repeated until both players have played someone else or waited a week. Andy would haunt the courts until a challenge was being played. Then, like Doc Holiday calling out Wyatt Earp from theLong Branch Saloon, he would insult his target player before the ball had stopped rolling on match point.

By default or play or ploy, Andy made his obnoxious way to the top. And there he stayed, in proverbial loneliness, because no one would challenge him.

Finally, there was Polly Jane from Chevefly. Here fame came from her use of that article of the code called the "free challenge pending" rule. According to this, every new player on a ladder can challenge anywhere, even up to No. 1, as an initial match. Moreover, a player can drop off a ladder for two weeks and return with a free challenge pending.

Polly would scout assorted ladders until she found one high-ranking player she could beat. Though almost all of the others could beat her, there was always one good player just tailormade for her pittypat game and her incessant lobbing and her girls-12-and-under serve.

So she would drop off the ladder and wait for her pigeon to soar to the top. Then she would drop in from her limbo, exercise her free challenge, and take over the top rung.

Inevitably she'd be beaten down, drop off the ladder, and wait for her private patsy to rise again. In her prime, Polly once came out of nowhere in eight calendar days to take the No. 1 ranking on six different ladders. And she never had to make a bad call to do it.