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'''[[Main Page|Home]] * [[Chess]] * [[Moves]] * Legal Move'''
[[FILE:The_King_Can_Never_Be_Captured.jpg|border|right|thumb|link=https://fineartamerica.com/featured/the-king-can-never-be-captured-marina-kalinovsky.html
| [[Arts#Kalinovsky|Marina Kalinovsky]] - The King can never be Captured <ref>[https://fineartamerica.com/featured/the-king-can-never-be-captured-marina-kalinovsky.html The King can never be Captured by Marina Kalinovsky, Artwork for Sale - Brooklyn, NY - United States]</ref> ]]
A '''Legal Move''' is a [[Pseudo-Legal Move|pseudo-legal move]] which does not leave its own [[King|king]] in [[Check|check]]. If not in check, most programs delay the legality test to the child node, after [[Incremental Updates|incremental updates]] [[Attack and Defend Maps|attack and defend maps]] or an explicit [[Square Attacked By|square attacked test]] direct after [[Make Move|make move]]. Earlier programs were even more "lazy", to determine illegal moves after an illegal king capture, while many programs consider [[Pin#AbsolutePin|absolutely pinned]] pieces in [[Move Generation|move generation]] and also for [[Evaluation|evaluation]] purpose, or even perform a strictly legal generation to demand searching legal positions. If in check, most programs apply a special move generator to omit the majority of illegal moves.
<span id="LegalityTest"></span>
=Legality Test=
Beside the pseudo-legal condition which might already be sufficient for a subset of chess programs, there are several conditions to test, dependent whether the king is in check or not.
==Not in Check==
* The moving piece is not [[Pin#AbsolutePin|absolutely pinned]] on its [[Direction|move direction]]
* [[En passant]] requires special horizontal pin test of both involved pawns, which disappear from the same [[Ranks|rank]]
* While [[Castling|castling]], [[Rook|rook]] is not pinned horizontally in [[Chess960]]
==King in Check==
===[[Double Check]]===
* Only king moves to non attacked squares, sliding check x-rays the king
===[[Check|Single Check]]===
* Capture of checking piece. The capturing piece is not [[Pin#AbsolutePin|absolutely pinned]]
* King moves to non attacked squares, sliding check x-rays the king
* Interposing moves in case of distant sliding check. The moving piece is not absolutely pinned.
=See also=
* [[DirGolem]]
* [[Hash Move]]
* [[Killer Move]]
* [[Mate Killers]]
* [[Move Generation]]
* [[Pseudo-Legal Move]]
* [[Square Attacked By#LegalityTest|Pseudo Legality Test]] with [[Bitboards]]
=Forum Posts=
* [http://www.open-aurec.com/wbforum/viewtopic.php?f=4&t=53063 Question about testing legality of moves] by ambrooks1, [[Computer Chess Forums|Winboard Forum]], February 07, 2014
* [http://www.open-chess.org/viewtopic.php?f=5&t=2697 En-passant legality test] by tetra, [[Computer Chess Forums|OpenChess Forum]], August 07, 2014 » [[En passant]]
* [http://www.talkchess.com/forum/viewtopic.php?t=63090 TTMove legality checking ? & Killers Move Format?] by [[Mahmoud Uthman]], [[CCC]], February 08, 2017 » [[Hash Move]], [[Killer Move]]
=References=
<references />
'''[[Moves|Up one Level]]'''
[[FILE:The_King_Can_Never_Be_Captured.jpg|border|right|thumb|link=https://fineartamerica.com/featured/the-king-can-never-be-captured-marina-kalinovsky.html
| [[Arts#Kalinovsky|Marina Kalinovsky]] - The King can never be Captured <ref>[https://fineartamerica.com/featured/the-king-can-never-be-captured-marina-kalinovsky.html The King can never be Captured by Marina Kalinovsky, Artwork for Sale - Brooklyn, NY - United States]</ref> ]]
A '''Legal Move''' is a [[Pseudo-Legal Move|pseudo-legal move]] which does not leave its own [[King|king]] in [[Check|check]]. If not in check, most programs delay the legality test to the child node, after [[Incremental Updates|incremental updates]] [[Attack and Defend Maps|attack and defend maps]] or an explicit [[Square Attacked By|square attacked test]] direct after [[Make Move|make move]]. Earlier programs were even more "lazy", to determine illegal moves after an illegal king capture, while many programs consider [[Pin#AbsolutePin|absolutely pinned]] pieces in [[Move Generation|move generation]] and also for [[Evaluation|evaluation]] purpose, or even perform a strictly legal generation to demand searching legal positions. If in check, most programs apply a special move generator to omit the majority of illegal moves.
<span id="LegalityTest"></span>
=Legality Test=
Beside the pseudo-legal condition which might already be sufficient for a subset of chess programs, there are several conditions to test, dependent whether the king is in check or not.
==Not in Check==
* The moving piece is not [[Pin#AbsolutePin|absolutely pinned]] on its [[Direction|move direction]]
* [[En passant]] requires special horizontal pin test of both involved pawns, which disappear from the same [[Ranks|rank]]
* While [[Castling|castling]], [[Rook|rook]] is not pinned horizontally in [[Chess960]]
==King in Check==
===[[Double Check]]===
* Only king moves to non attacked squares, sliding check x-rays the king
===[[Check|Single Check]]===
* Capture of checking piece. The capturing piece is not [[Pin#AbsolutePin|absolutely pinned]]
* King moves to non attacked squares, sliding check x-rays the king
* Interposing moves in case of distant sliding check. The moving piece is not absolutely pinned.
=See also=
* [[DirGolem]]
* [[Hash Move]]
* [[Killer Move]]
* [[Mate Killers]]
* [[Move Generation]]
* [[Pseudo-Legal Move]]
* [[Square Attacked By#LegalityTest|Pseudo Legality Test]] with [[Bitboards]]
=Forum Posts=
* [http://www.open-aurec.com/wbforum/viewtopic.php?f=4&t=53063 Question about testing legality of moves] by ambrooks1, [[Computer Chess Forums|Winboard Forum]], February 07, 2014
* [http://www.open-chess.org/viewtopic.php?f=5&t=2697 En-passant legality test] by tetra, [[Computer Chess Forums|OpenChess Forum]], August 07, 2014 » [[En passant]]
* [http://www.talkchess.com/forum/viewtopic.php?t=63090 TTMove legality checking ? & Killers Move Format?] by [[Mahmoud Uthman]], [[CCC]], February 08, 2017 » [[Hash Move]], [[Killer Move]]
=References=
<references />
'''[[Moves|Up one Level]]'''
