You know how to play checkers. But you don't know how to play checkers against him.

Nobody has beaten the checker champion in a match for 30 years. Even computer programmers don't take him on, despite a $5,000 challenge from a checker organization.

"His style is invincibility," says Burke Grandjean, secretary of the American Checker Federation in Baton Rouge, La. "There's no sport - if you can call this a sport - that's ever had a champion like him. No one can touch him."

That kind of praise is usually reserved for big-name boxers, football players, tennis pros, sometimes even chess players. But who can name the world checker champion?

Pay attention: His name is Marion Tinsley. He is 58 years old. He teaches mathematics at Florida A&M University in Tallahassee and Bible classes at a church in nearby Perry. And he is the reigning king of the checkerboard, worldwide.

The king is a gentle, soft-spoken, modest man. Ask him about the longest game he has ever played and he says it lasted seven hours and 30 minutes and ended in a draw with Derek Oldbury of Great Britain. What he doesn't say is that the marathon game in 1958 set a world record for length.

"You don't see many conceited checker players," says Tinsley. "Almost every player has been told: "Oh, my kid plays checkers. You ought to play him; he always beats me." That's the ultimate put-down. As a result, checker players tend to be a little humble, because they realize the world does not think that much of them."

Advertisers don't ask checker champions to endorse their products; People magazine doesn't ask them for interviews; and network television doesn't broadcast their tournaments. If those things are the measure of society's recognition, then the low profile is OK with them. They're not in it for fame or money - just for the game.

Tinsley first won the world title, determined by the American Checker Federation, in 1955, and except for a temporary retirement from competition, he has never been dethroned.

One man will challenge the champion next month. Asa Long, holder of the world title from 1934 to 1948 and current American Tourney champion, wants to regain the honor when he and Tinsley square off in the world-title match at the International Checker Hall of Fame in Petal, Miss. Two years ago, Tinsley defeated him.

Long, 80, a retired machinery worker for the General Tire and Rubber Co., has been a mentor to Tinsley since the early 1940s.

"Marion and I are good friends," says Long from his home in Toledo, Ohio. "We know so much about each other's style and play that sometimes we just know the game will be a draw because we've played it before."

When the two friends play for the world title, "In many ways it will be like two artists collaborating on a work of art," says Tinsley. "There's a lot of beauty in checkers."

It's the elegance of such a deceptively simple game that draws Tinsley and other players into its intricacies. Checkers suffers from its bucolic image. It has long been considered the pastime of yokels sitting on cracker barrels at the country store. Or a kids' game. Or a déclassé form of chess. "Chess is the game of the classes and checkers is the game of the masses," wrote the late checker expert William Ryan.

At Tinsley's level, though, checkers is an intellectual challenge. Its early forms were pursued by the ancient Egyptians and Greeks. Upstairs in the den of his country home, just south of Tallahassee, Tinsley keeps about 200 books about checkers - a small collection, he says. One 45-year-old volume, Ryan's Modern Encyclopedia of Checkers, has been read so much that he had to have it rebound. It's his checkers bible, full of checker moves interspersed with anecdotes, quips and poems about checkers. He has penciled in notes and corrections on almost every page. Another book, Checkers: The Tinsley Way, was written by admirer Robert Shuffett in 1982.

Tinsley is hard put, or too modest, to define his own style of play. Grandjean, of the American Checker Federation, describes it with admiration: "For one, he has a terrific memory, an eidetic memory, like a photographic memory. Besides that, he has the ability to picture and look ahead 30 moves and accurately portray a board position. Also his temperament: He's calm and doesn't get excited, although he says he gets butterflies before a game, but you could never tell.

"If (chess champion Bobby) Fischer is erratic and flamboyant, then Tinsley is the exact opposite: very quiet, very subdued."

No checker player in Tallahassee or anywhere thereabouts is good enough to keep up with Tinsley. He is unmarried; his mother, Viola, 92, lives with him, but she has never learned to play checkers. Sometimes friends fly into town for games. The scarcity of opponents doesn't bother him. "I guess I love the game more than the players," he says.

When the two friends play for the world title, "In many ways it will be like two artists collaborating on a work of art," says Tinsley. "There's a lot of beauty in checkers."

On a cluttered table in his den, beneath a ceiling fan, Tinsley's custom-made checkerboard is set up. The 64 squares are of green-and-buff tiles - easier on the eyes than red and black. The two dozen checker pieces are red and white. He also keeps a magnetic checkerboard at his bedside to work on problems when he wakes up during the night. The boards are almost superfluous, though, because Tinsley devises most of his checker strategy and makes most of his moves entirely in his head.

As an Ohio State University student, he spent eight hours a day working on checkers - studying and analyzing moves, learning openings and memorizing plays. Now he has cut his study time to an hour a day, but "hardly a week goes by when I don't discover something new. Out of the clear blue sky an improvement of a published play will just come to mind, as if the subconscious has been working to come to light. A lot of my discoveries come that way, out of the clear blue sky. Some of my insights into the Scriptures come the same way."

Painters, musicians, writers and scientists have described their inspirations in similar fashion. Indeed, some checker players "compose" games the way a musician composes music. The Encyclopedia of Checkers goes so far as to declare: "Marion Tinsley is to checkers what Leonardo da Vinci was to science, what Michelangelo was to art, and what Beethoven was to music."

Tinsley can't remember exactly when he learned to play checkers or who taught him, but as a child he played with his father, his brother and a boarder named Mrs. Kershaw in their Columbus, Ohio, home. At that point he knew only the arithmetic of checkers. Mrs. Kershaw "used to beat me and rub it in and laugh and laugh," Tinsley recounts. "Oh, how she'd cackle as she'd jump my men."

A few years later, when he was 15 and looking for a geometry book in the Ohio State University Library, Tinsley came across Winning Checkers, an elementary book by Millard Hopper. That's when he discovered the game's complexities, its algebra.

The rest of the checker world was soon awed by the Ohio State math student who started beating the checker greats. In 1955, at age 28, Tinsley won the world title against Walter Hellman, his friend and mentor. The trophy, about 4 feet tall, stands on the floor in Tinsley's den with nine other trophies, haphazardly arranged.

In 1957, Tinsley received his doctorate in mathematics from Ohio State. A year later, he moved to Tallahassee to teach at Florida State University, and he successfully defended his world-champion checkers title. Thereupon he retired from competition, so that he could devote all his time to researching combinatorial analysis, a field of mathematics concerned with arranging the objects of a set into patterns.

"I ascended the checker ladder and I had no more things to conquer," he says. "The man I beat for the world title came back and beat everyone else again." That man, Walter Hellman, died in 1975.

Tinsley had no plans to return to competition. He no longer studied the game often, and he put away his bedside checkerboard. Mathematics and checkers shared so many similarities that giving up one for the other seemed an even trade.

During his hiatus from checkers, Tinsley became a part-time minister, volunteering at the Church of Christ, Tallahassee's first integrated church. He also continued to teach combinatorial analysis, abstract algebra, number theory and calculus. In 1968, he transferred to Florida A&M, a predominantly black state university, so that he could teach more black students.

Later he joined the charismatic religious movement and now teaches from the Book of Revelation in a weekly Bible class. When Tinsley studies checkers he listens to Bach, Handel, Brahms and Grieg, but when he's just relaxing, he may listen to spirituals.

Tinsley was content with his math and religious studies, but a friend in Kentucky kept urging him to make a checker comeback. "Finally I made a deal with him," says Tinsley. "I'd compete again if he'd give up drinking. I played again, but he didn't give up drinking, although he did gain quite a victory over it."

Tinsley won the U.S. checkers championship in 1970. In 1975, he regained the world title and has held it ever since.

Come May 20, Tinsley goes up against Long. The atmosphere in the International Checker Hall of Fame will be tense during the two-week tournament. The opponents will play as many as 40 games. Each game can be agonizingly slow, moving along at 20 moves an hour. The player who wins the most games takes the title and about $5,000 from the American Checker Federation. The money is incidental.

In preparation, Tinsley is doing little more than devoting his usual hour a day to studying checkers. "When I was younger," he says, "I used to take weights to tournaments and matches, and I would take classical music to help me relax at night. I don't do that anymore. As I get older, I have a little more control over the stress. My Christian faith has helped me with that, and I didn't have that in college."

Tinsley is the favorite in this match. The king of the checkerboard is expected to win again. But he is also just a little insecure, which, he reasons, is "probably helpful for a checker player. Insecurity makes him more cautious."