The Poisoned Square
One of the things we do when we analyze a chess postion is look for patterns; things like pinned pieces, knight forks, pawn forks and the like. Its always important to look for them so we don't accidently miss them when they actually come up. Here is an example of a simple knight fork pattern:
If a knight should land on e3 the queen and rook are immediately forked; even though this fork is not readily available at the moment, the potential is still there and it is a possible weakness that can be exploited (if the pawn on f2 should be advanced and the queen bishop fianchettoed for example).
This used to be the extent of my pattern searches until I studied a particular GM game and I saw a whole new kind of pattern being exploited; it is a hidden type of pattern that is almost invisible if you aren't looking for it.
I'm sure that this has been written about in various books, but for me it was the first time I had seen it and so I came up with a name for it: The Poisoned Square.
Of course everybody has heard of the poisoned pawn, where if you capture it, your opponent is able to obtain a decisive advantage; the poisoned square is a similar concept: if the opponent's target piece enters this square, a (potential) decisive advantage will be gained immediately.
Here is one example from the same position:
To determine which squares are poisoned you need to have two targets, usually this will be the queen and king, a rook and the king, a rook and the queen or two rooks. In the above case we have the king and queen as targets. The squares in red are immediately accessible to the queen and are poisoned because of their relationship to the king: e1 and d2 are poisoned because of a potential knight fork on f3, d4 is poisoned because of a potential knight fork on e2(!).
Its always nice when an enemy piece is on a posioned square as in the above with the pawn on d4 and with the search for posioned squares in this position a tactical idea has emerged: If a knight should be on c3, the pawn on d4 would no longer be protected and would be free for capture.
Similar analyses could be conducted for the other major pieces as well (a2 is poisoned for instance).
The game continued:
The amazing thing is that I used to miss these potential exploitations entirely because I didn't know how to look for them!
I believe that poisoned square possibilities abound in our chess games and so I thought I would pass this on in case it might help someone else.
Also, if someone has encountered this in a book elsewhere and it has a different name, feel free to share.