Boris Spassky once had it but lost it. Victor Korchnoi barely missed winning it three years ago. This week in Yugoslavia, the two Russians in exile will begin a long, grueling match to determine which of them wins another crack at the world chess championship.

Three years ago, in Russia, Korchnoi and Anatoly Karpov played a similar match - the final in a series of candidates' matches to select a challenger for the enigmatic world champion of that time, Bobby Fischer. Karpov won the 23-game match by a single point, and later, when Fischer declined to defend his title, the Karpov-Korchnoi match became, in retrospect, the de facto world championship. According to rumors circulating internationally among chess players, the Korchnoi-Spassky match could also turn out to be a world championship match by default.

The rumors say that if Korchnoi wins, the Soviet Chess Federation will refuse to allow Karpov to play him. Korchnoi became a nonperson in Russia in July, 1976, when he won the IBM international tournament in Amsterdam and then asked the Dutch government for political asylum. After a brief round of bitter denunciation, the Russian press began to give him the silent treatment and the Soviet Chess Federation tried to knock him out of the world championship competition by political maneuvering. When this failed, the Russians declared that no Russian player would compete in a chess event in which Korchnoi participated. He was pitted against two Russians in a row in the world championship preliminaries: Tigran Petrosian in the quarter-finals and Lev Polugaevsky in the semi-finals, and the Soviet chess bureaucracy allowed both to play him, since the alternative was to lose the matches by forfeit. Korchnoi won devastating victories over both opponents. In a sense, it would have been better for the Russians to forfeit, and they could conceivably take that option next year if they are confronted with the prospect of a Karpove-Jorchnoi match for the world title.

There would be a certain poetic justice in that; Karpov would lose the world championship as he won it, without pushing a single pawn in a championship match. But the odds are that, faced with such a decision, the Soviets would choose as they have done before, to bend their "principles" and take a chance at winning rather than face a sure loss by forfeit. Bobby Fisher demonstrated that he would throw away a world chess championship rather than compromise. But he is probably the only one who would.

In addition, the Russians have every reason to believe that Karpov would be at Korchnoi. He did it three years ago, not overwhelmingly but adequately. Since becoming champion, he has complied an impressive record, finishing first in nearly every tournament he has played and losing only five games. A look at the record supports the claim that he is the world's strongest active player.

Meanwhile, before the question of Korchnoi vs. Karpov can be brought up, there remains the immediate question of Korchnoi vs. Spassky. The two have played one previous match, nearly 10 years ago, when Spassky beat Krochnoi en route to winning the world title in 1969. The results of that long-ago match, however, have little relevance to the one coming up; the current track record indicates, and Spassky has publicly agreed, that Korchnoi is a much stronger player now than he was then.

Spassky is harder to diagnose. When playing in top form, he is one of the half-dozen greatest players in chess history, but he is as moody and volatile as any character in a Dostoevksy novel, and when his mood is down he loses. His semifinal match with Hungarian grandmaster Lajors Protisch, exemplifying this unevenness, included some of the best and some of the worst games of his career. It was seesaw struggle, with Spassky falling behind twice in the first part of the match and finally pulling himself together with a couple of brilliant victories. He got into the world championship cycle this time only as an alternate, because one of the eligible players was unable to compete in a qualifying tournament. From this shaky beginning, he has made an uneasy progress to the final candidates' match - but there he is.

The Russians would undoubtedly prefer Spassky to win the match that begins this week, given the necessity of choosing between these two players; but if they had their choice, neither contender would be trying for a shot at Karpov. Unlike Korchnoi, who has burned all his bridges behind him, Spassky is living outside the Soviet Union with grudging government consent. Considering the alternative, the Soviet press has treated Spassky well, printing full and often annotated scores of all his recent games, while the Korchnoi victories that must be mentioned get three-line notices buried in inconspicuous corners.

Whatever the outcome of the upcoming match, the odds are that it will produce a lot of exciting chess. Korchnoi and Spassky both play a complex tactical brand of chess, with exciting intricate combinations, and the latest evidence is that both are in excellent form. Two pieces of that evidence are presented below: the first form a game played by Korchnoi in Germany's very strong national chess league; the second from a recent training match with Dutch grandmaster Jan Timman, which Spassky won by a margin of two victories and four draws.