I go to thinking a little while ago -- during the flight home, actually -- about the classic chess denouement that is almost a stereotype in the popular (i.e., Hollyweird) culture. We've seen it many times over: Player A -- who is to be the goat of this little morality play -- moves a piece and announces check to Player B's king, typically with a look of smug self-assurance. A few seconds later, the script calls for Player B to respond with a move and announce -- to our great delight, presumably -- checkmate to Player A's king.
Just how likely is it for Player A to check and have Player B respond with checkmate?
Anyone familiar with the game will tell you that there are three ways to parry a check, and that if none of the methods work, the side whose King is threatened is, in fact, checkmated. These three ways are:
(a) to move the King away from the line of attack. Such a move could conceivably
discover (i.e., reveal) a check against the attacker's King, but such a "counter" check
could not be checkmate, since the original attacking piece could move to the square originally occupied by the defending King, blocking the immediate threat.
(b) to move a piece between an attacking piece and a checked King, which is called
interposition. In all cases where this is possible, the defender's interposed piece comes under attack by the piece that was checking the King. Therefore, if the interposing piece does threaten to capture the attacker's King, such a threat
could not be checkmate, since the interposing piece could be captured, again blocking the immediate threat.
(c) to capture the attacking piece. This is the
only alternative that
will allow a checkmate to occur immediately after a check, but it would require a fairly severe case of chess "blindness" on the part of a player not to see that the attack could be refuted in grand style with a capture... that mates your own King!
Why did I write this? I don't know... I think to get it off my chest.
Cheers...
UPDATE: In commenting on this freewheel, LJ friend
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avva pointed out the following:
1) In either a) or b) the original attacking piece could be pinned.
2) In b), the defending piece could be blocking a different line of attack, by a different piece, on the original player's king; when it is moved to defend from the check, the line of attack is revealed to be checkmate.
3) A fancier twist on 2) is the possibility of the defending piece's movement opening a double check (that also happens to be a checkmate) on the original player's king, thus rendering the ability of the original attacking piece to capture the defending piece, or to defend against the other piece's line of attack, insufficient.
To which I replied:
Excellent! I hadn't thought of case 2 (or 3, which is 2 on steroids).
However, for case 1, if the original attacking piece is pinned, then the check can be parried by having the pinning piece capture the attacker... unless of course, the pinning piece is itself pinned, so it can't move.
Hmmm. In that case, we would have a check by A, defended by an interposition by B that checks A's King, followed by an interposition by A that checks B's King, which moves, discovering checkmate against A's King.
Four checks in a row, culminating in mate. Now that would be something to see!
Thanks for the response.
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