Unclear : 1977 Korchnoi - Spassky

In the post Unclear Positions, I wrote,

I have a tall stack of old, pre-Chessbase Informants standing in the corner of my study. It might be interesting to go back and look for positions tagged by the annotator as '∞'. Depending on what I discover, it might even be the start of a new series of posts on this blog.

A typical Informant from that period had 600-700 games, so the first problem is 'Where to start?' I decided that fertile ground would be games having something to do with the World Chess Championship, a subject that I'm always eager to learn more about.

For my first post in the series I tackled the Korchnoi - Spassky 1976-78 Candidates Final Matches played November-December 1977, in Belgrade. Of the 18 games in Informant 24 (INF), 16 were annotated by Michael Stean and the other two by Raymond Keene. Both players served as Korchnoi's seconds during the match. In the notes I found eight positions marked '∞', all but one of which were in unplayed variations. The exception followed from this diagram.

1977 Candidates Final Match (game 7)
Spassky, Boris

Korchnoi, Viktor
(After 18...Nc5-e6)
[FEN "r2r2k1/1b3pp1/1p1qnb1p/pP1p4/3N2B1/2N1P3/P4PPP/2RQR1K1 w - - 0 19"]

The game continued 19.Bxe6 (INF [Stean]: '!?') 19...fxe6. Now on 20.Nc6 (INF: '!'), we reach the mysterious 'unclear (∞)' position. After 20...Bxc6 21.bxc6, as in the game, the tactical point is 21...Qxc6 22.Ne4 Qd7 23.Nxf6+ gxf6 24.Qd4.

Spassky tried 21...Bxc3 and lost to a brilliant series of tactical blows where both players had Pawns on the seventh rank. After 21...Qb4 (INF: '!?'), Stean gave 22.a3 Qxa3 23.c7 Rdc8 24.Nb5 'with compensation'. Rybka prefers 21...Qxc6.

To play through the complete game see...

Viktor Korchnoi vs Boris Spassky, Belgrade cm f 1977
http://www.chessgames.com/perl/chessgame?gid=1082385

...on Chessgames.com.