Editing Chess960 (section)

==Castling rules==
As in classical chess, each player may [[castling|castle]] once per game, moving both the king and a rook in a single move; however, the castling rules were reinterpreted in Chess960 to support the different possible initial positions of king and rook. After castling, the final positions of king and rook are the same as in classical chess, namely:

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|+ Examples of castling
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 |An example initial position of kings and rooks
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 |White has castled a-side (0-0-0) and Black has castled h-side (0-0).
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* After ''a-side'' castling ({{chessgloss|queenside}}/long castling in classical chess), the king finishes on the c-{{chessgloss|file}} and the a-side rook finishes on the d-file. The move is notated 0-0-0 as in classical chess.
* After ''h-side'' castling ({{chessgloss|kingside}}/short castling in classical chess), the king finishes on the g-file and the h-side rook finishes on the f-file. The move is notated 0-0 as in classical chess.

Castling prerequisites are the same as in classical chess, namely:
* The king and the castling rook must not have previously moved. If the king did not move while castling on a previous move (i.e. the king is on c1 or c8 already while castling a-side or on g1 or g8 already while castling h-side), it may be possible for this condition to still hold for castling on the other side. However, the FIDE rules explicitly state that castling may be done only once per game per player.
* No square from the king's initial square to its final square may be under attack by an enemy piece, even if the king is already on its final square.<ref>{{cite news |url=https://chess24.com/en/read/news/fischer-random-2-wesley-so-tries-to-castle-illegally |title=Fischer Random 2: Wesley So tries to castle illegally |website=[[chess24]] |archive-url=https://web.archive.org/web/20221027163457/https://chess24.com/en/read/news/fischer-random-2-wesley-so-tries-to-castle-illegally |date=October 27, 2022 |archive-date=2022-10-27}}</ref>
* All the squares between the king's initial and final squares (including the final square), and all the squares between the castling rook's initial and final squares (including the final square), must be vacant except for the king and castling rook.<ref>{{cite web |url=http://www.dwheeler.com/essays/Fischer_Random_Chess.html |title=Fischer Random Chess (Chess960) |website=www.dwheeler.com}}</ref>

FIDE's recommended procedure for castling unambiguously is first to move the king outside the playing area next to its final square, then to move the rook to its final square, then to move the king to its final square. Another recommendation is to verbally announce the intent to castle before doing so.<ref name="FideLawsOfChess">{{cite web |title=Fide Laws of Chess taking effect from 1 January 2023 |url=https://handbook.fide.com/chapter/E012023 |publisher=[[FIDE]]}}</ref>

===Observations===
* In some starting positions, squares can remain occupied during castling that would be required to be vacant under standard rules. Castling a-side (0-0-0) could still be possible despite the home rank a-, b-, or e-file squares being occupied, and similarly for the e- and h-files for h-side castling (0-0). In other positions, it can happen that one of the king or rook does not move during the castling maneuver since it already occupies its destination square – e.g., an h-side rook that starts on the f-file; in this case, only the king moves. No initial position allows a castling where neither piece moves, as the king must start between the rooks.
* Another unusual possibility is for castling to be available as the first move of the game, as happened in the 11th game of the tournament match between [[Hikaru Nakamura]] and [[Magnus Carlsen]], Fischer Random Blitz 2018. The starting position had kings at f1/f8 and h-side rooks at g1/g8. Both players took the opportunity to castle on the first move (1.0-0 0-0).<ref>{{cite web |url=https://en.chessbase.com/post/chess960-day-5 |title=Carlsen adds a new title: Chess960 champion |publisher=[[ChessBase]] |date=February 14, 2018 }}</ref>
* In standard chess, a rook can castle out of and through a threatened square but cannot castle into a threatened square. While there are no rules specifying that a rook cannot castle into a threatened square, they are naturally unable to. This is because a castling king will pass through the square the castling rook ends up on and the rules prohibit a king to pass through threatened squares during castling. In Chess960, however, the relative position of the King and Rooks sometimes allows for a rook to legally end up on a threatened square. This is discussed by [[Levon Aronian]] and [[Yasser Seirawan]] in Round 3 of St Louis' 2022 Chess 9LX tournament for move 18 of Aronian's game against [[Leinier Domínguez Pérez]] which had starting position RKBBNQNR.<ref>{{citation |url=https://www.youtube.com/watch?v=p2CjJa78c24 |title=Aronian: Beating Kasparov Day 1 |website=[[YouTube]] |date=September 14, 2022 }}</ref><ref>{{citation |url=https://www.youtube.com/watch?v=LIwinEQ9KFQ |title=Levon mansplains 9LX castling to Yasser. (2022Sep) |website=[[YouTube]] |date=November 11, 2022 }}</ref><ref>{{citation |url=https://lichess.org/broadcast/2022-champions-showdown-chess-9lx/round-3/RKKa66zy/L8FqHntz#35 |title=2022 Champions Showdown: Chess 9LX Round 3: Aronian, Levon - Dominguez, Leinier |website=[[Lichess]] |date=September 14, 2022 }}</ref>
* There are exactly 90 starting positions where, unlike in standard chess, players have to give up castling rights on one side in order to castle on the other side. This is seen by calculating that this happens 18 times in each of five possible groups of starting positions namely RKRXXXXX, RKXRXXXX, XRKRXXXX, XXXXXRKR and RXKRXXXX. In only these positions, a rook has to be moved (or captured) on one side in order to castle on the other side.<ref name="chess.stackexchange.com">{{Cite web|url=https://chess.stackexchange.com/questions/37082/how-many-chess960-positions-exist-in-which-castling-on-one-side-does-not-require|title = Chess Stackexchange – How many Chess960 positions exist in which castling on one side does not require moving the rook on the other side?}}</ref> For example, in the starting position RKRBBNNQ, which is in the first group RKRXXXXX, a player intending to castle a-side must first move the c-file rook (or let it be captured).
** The Sesse evaluations<ref name="Sesse 1">{{Cite web|url=https://www.chess.com/article/view/whats-the-most-unbalanced-chess960-position|title=What's The Most Unbalanced Chess960 Position?|first=Mike|last=Klein (MikeKlein)|website=Chess.com|date=March 14, 2018 }}</ref><ref name="Sesse 2">{{Cite web|url=https://docs.google.com/spreadsheets/d/1JVT6_ROOlCTtMmazzBe0lhcGv54rB6JCq67QOhaRp6U/edit#gid=0|title = Fischer Random – 960 startposisjoner}}</ref> (which used [[Stockfish (chess)|Stockfish 9]]) show that White has about, on average, a 7% increased advantage in these 90 positions (Evaluation is 0.1913) compared to the remaining 870 positions (Evaluation is 0.1790).<ref name="ReferenceA">{{Cite web|url=https://chess.stackexchange.com/questions/38722/what-is-whites-increased-advantage-in-chess90-as-compared-to-chess870-chess96|title = Chess Stackexchange – What is white's increased advantage in chess90 as compared to chess870? (Chess960 can be split into 2 subsets, chess90 and chess870)}}</ref>