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'''[[Main Page|Home]] * [[Chess]] * [[Tactics]] * Double Attack'''
[[FILE:Le duelliste à l'épée et au poignard.jpg|border|right|thumb|Le duelliste à l'épée et au poignard <ref>[[Arts#Callot|Jacques Callot]] - [http://commons.wikimedia.org/wiki/File:Le_duelliste_%C3%A0_l%27%C3%A9p%C3%A9e_et_au_poignard.jpg Le duelliste à l'épée et au poignard], 1621-1625, Source: [http://gallica.bnf.fr/ark:/12148/btv1b8495739c/f1.item Les Gobbi, Le duelliste à l'épée et au poignard : estampe / Jacques Callot], [https://en.wikipedia.org/wiki/Biblioth%C3%A8que_nationale_de_France Bibliothèque nationale de France], [http://commons.wikimedia.org/wiki/Category:Jacques_Callot Category:Jacques Callot] from [https://en.wikipedia.org/wiki/Wikimedia_Commons Wikimedia Commons]</ref> ]]
A '''Double Attack''' attacks two or more pieces or important squares simultaneously with one [[Moves|move]], either by [https://en.wikipedia.org/wiki/Fork_%28chess%29 forking] with the moving piece, or at least by a single direct attack of the moving piece in conjunction with a [[Discovered Attack|discovered attack]]. The so called royal knight fork winning the queen is most important, but like other double attacks involving [[Check|checks]] already covered by usual move selection heuristics.
=Seeing Potential Forks=
Like a [[Skewer|skewer]] or other [[Tactical Moves|tactical threats]], it might be nice to determine the availability of forks in advance, to either try those moves early, to don't [[Pruning|prune]] or [[Reductions|reduce]] them, or even to try them at the [[Horizon Node|horizon]] in [[Quiescence Search|quiescence search]]. As always, it depends on the [[Board Representation|board representation]] and the availability of appropriate data structures like [[Attack and Defend Maps]], whether additional effort in determination of fork pattern, what kind of pieces to consider etc., makes sense and pay off.
==Knight and Pawn==
A common technique in [[Bitboards|bitboards]] to determine fork move target sets for respective pieces is to treat potential opponent targets as the kind of piece which may fork, and to pairwise [[General Setwise Operations#Intersection|intersect]] all their disjoint direction attacks, as demonstrated in [[Knight Pattern#KnightForks|knight forks]].
Pawn fork targets have only two different and contact attacking directions. For instance, white pawn fork targets need the intersection of east and west black [[Pawn Attacks (Bitboards)|pawn attacks]] of all black pieces, excluding the guarding real black pawn attacks. If this resulting set intersects with white [[Pawn Pushes (Bitboards)|pawn push targets]], white has likely (considering [[Pin|pins]]) one or more pawn forks.
Single pawn and knight forks are easy to determine with any board representation, since the patterns are simple. For knight forks one may rely on [[Knight-Distance|knight-distance]] of two, implying the precondition of [[Color of a Square#SameColor|same square color]] between all potential attacking targets and one own attacking knight.
==Sliding Pieces==
For [[Sliding Pieces|sliding piece]] forks, [[Mailbox]] or [[0x88]] based board representations may rely on techniques as described in [[Intersection Squares]], while it seems bitboards have an edge using attack set intersection of a pair of targets.
=See also=
* [[Knight Pattern#KnightForks|Knight Forks]]
* [[Discovered Attack]]
* [[Discovered Check]]
* [[Double Check]]
* [[Skewer]]
=External Links=
* [https://en.wikipedia.org/wiki/Fork_%28chess%29 Fork (chess) from Wikipedia]
* [https://en.wikipedia.org/wiki/Discovered_attack Discovered attack from Wikipedia]
* [http://www.top10chess.com/2008/09/tactical-motif-i-double-attack.html Top 10 chess: Tactical Motif I: Double Attack]
* [http://www.chesstactics.org/index.php?Type=page&Action=none&From=2,1,1,1 The Double Attack] from [http://www.chesstactics.org/ Ward Farnsworth's Predator at the Chessboard]
* [http://www.mark-weeks.com/aboutcom/aa03d05.htm Middle game - Double attacks] by [[Mark Weeks]]
=References=
<references />
'''[[Tactics|Up one Level]]'''
[[FILE:Le duelliste à l'épée et au poignard.jpg|border|right|thumb|Le duelliste à l'épée et au poignard <ref>[[Arts#Callot|Jacques Callot]] - [http://commons.wikimedia.org/wiki/File:Le_duelliste_%C3%A0_l%27%C3%A9p%C3%A9e_et_au_poignard.jpg Le duelliste à l'épée et au poignard], 1621-1625, Source: [http://gallica.bnf.fr/ark:/12148/btv1b8495739c/f1.item Les Gobbi, Le duelliste à l'épée et au poignard : estampe / Jacques Callot], [https://en.wikipedia.org/wiki/Biblioth%C3%A8que_nationale_de_France Bibliothèque nationale de France], [http://commons.wikimedia.org/wiki/Category:Jacques_Callot Category:Jacques Callot] from [https://en.wikipedia.org/wiki/Wikimedia_Commons Wikimedia Commons]</ref> ]]
A '''Double Attack''' attacks two or more pieces or important squares simultaneously with one [[Moves|move]], either by [https://en.wikipedia.org/wiki/Fork_%28chess%29 forking] with the moving piece, or at least by a single direct attack of the moving piece in conjunction with a [[Discovered Attack|discovered attack]]. The so called royal knight fork winning the queen is most important, but like other double attacks involving [[Check|checks]] already covered by usual move selection heuristics.
=Seeing Potential Forks=
Like a [[Skewer|skewer]] or other [[Tactical Moves|tactical threats]], it might be nice to determine the availability of forks in advance, to either try those moves early, to don't [[Pruning|prune]] or [[Reductions|reduce]] them, or even to try them at the [[Horizon Node|horizon]] in [[Quiescence Search|quiescence search]]. As always, it depends on the [[Board Representation|board representation]] and the availability of appropriate data structures like [[Attack and Defend Maps]], whether additional effort in determination of fork pattern, what kind of pieces to consider etc., makes sense and pay off.
==Knight and Pawn==
A common technique in [[Bitboards|bitboards]] to determine fork move target sets for respective pieces is to treat potential opponent targets as the kind of piece which may fork, and to pairwise [[General Setwise Operations#Intersection|intersect]] all their disjoint direction attacks, as demonstrated in [[Knight Pattern#KnightForks|knight forks]].
Pawn fork targets have only two different and contact attacking directions. For instance, white pawn fork targets need the intersection of east and west black [[Pawn Attacks (Bitboards)|pawn attacks]] of all black pieces, excluding the guarding real black pawn attacks. If this resulting set intersects with white [[Pawn Pushes (Bitboards)|pawn push targets]], white has likely (considering [[Pin|pins]]) one or more pawn forks.
Single pawn and knight forks are easy to determine with any board representation, since the patterns are simple. For knight forks one may rely on [[Knight-Distance|knight-distance]] of two, implying the precondition of [[Color of a Square#SameColor|same square color]] between all potential attacking targets and one own attacking knight.
==Sliding Pieces==
For [[Sliding Pieces|sliding piece]] forks, [[Mailbox]] or [[0x88]] based board representations may rely on techniques as described in [[Intersection Squares]], while it seems bitboards have an edge using attack set intersection of a pair of targets.
=See also=
* [[Knight Pattern#KnightForks|Knight Forks]]
* [[Discovered Attack]]
* [[Discovered Check]]
* [[Double Check]]
* [[Skewer]]
=External Links=
* [https://en.wikipedia.org/wiki/Fork_%28chess%29 Fork (chess) from Wikipedia]
* [https://en.wikipedia.org/wiki/Discovered_attack Discovered attack from Wikipedia]
* [http://www.top10chess.com/2008/09/tactical-motif-i-double-attack.html Top 10 chess: Tactical Motif I: Double Attack]
* [http://www.chesstactics.org/index.php?Type=page&Action=none&From=2,1,1,1 The Double Attack] from [http://www.chesstactics.org/ Ward Farnsworth's Predator at the Chessboard]
* [http://www.mark-weeks.com/aboutcom/aa03d05.htm Middle game - Double attacks] by [[Mark Weeks]]
=References=
<references />
'''[[Tactics|Up one Level]]'''
