Game 21 of the world chess championship was adjourned yesterday in Lyons, France, in a sharp, double-edged position where a small slip could mean disaster for either challenger Anatoly Karpov or defending champion Gary Kasparov.

Karpov wrote down and sealed his 41st move in an unenviable position. Kasparov is threatening 41. ... Ra1ch; 42 Kc2, Qa4, with a strong attack. In the closing moves of yesterday's session, however, the challenger had a chance to play for a draw by repetition of moves and turned it down by playing 39. Qf2 rather than 39. Kc2.

At this point in the match of course, draws are almost as bad for Karpov as losses. With the match score at 11 to 9 in Kasparov's favor, Kasparov can guarantee a 12-12 tie and secure his title for another three years by winning one game or drawing two. Then a draw, giving him 12 1/2 points, would entitle him to a $1.7 million first prize (rather than $1.5 million for a drawn match) and a gem-studded trophy valued at about $1 million. To regain the title he lost to Kasparov in 1985, Karpov must at least win three games and draw one. A maximum of four games remain in the match, including the one in progress.

Whatever happens when play is resumed today, it should be dramatic.

In Game 21, playing black against Karpov's queen's pawn opening, Kasparov once again chose the King's Indian Defense. This time, Karpov played what many experts consider the sharpest system in this opening, the Saemisch Variation. This was played in the first match game in New York; Kasparov went into the hypermodern Byrne Variation and Karpov nearly won.

This time, Kasparov, perhaps for the first time, played the classical line with 6. ... e5, then created a new idea on moves 10 and 11, based on the activization of his light pieces at the cost of some weakness on the black squares. Karpov had his pieces better placed than Kasparov but could not find any active play, so he simply waited (moves 16-18) while Kasparov shifted his knight from the kingside rim to a better square on c5.

Being a player who strives for initiative, Kasparov could not pass up the opportunity for 24. ... Bn5 trying to open lines to Karpov's king.

The two knights in the center of the board were bothering Karpov and, with 29. Bxd4, he tried to take advantage of the weak pawn on b5. It was clear that Kasparov should have avoided all the exchanges. But when Karpov's king at last found its way to relative safety, Kasparov quickly sacrificed a pawn with 34. ... b4, opening the a-file and leading to the adjourned position.

Kavalek is an international grandmaster; McLellan is a Washington Post staff writer. For results of the latest World Chess Championship game, call Post-Haste at 202-334-9000 on a Touch-Tone phone and enter code 8000. The service is free.