Castling is a special move in the game of chess involving the king
and either of the original rooks of the same colour. It is the only
move in chess (leaving aside promotion) in which a player moves
two pieces at the same time. Castling consists of moving the king
two squares towards a rook on the player's first rank, then moving
the rook onto the square over which the king crossed. Castling can
only be done if the king has never moved, the rook involved has
never moved, the squares between the king and the rook involved
are not occupied, the king is not in check, and the king does not
cross over or end on a square in which it would be in check.
The notation for castling, in both the descriptive and the algebraic
systems, is 0-0 with the kingside rook and 0-0-0 with the queenside
rook. In PGN, O-O and O-O-O are used instead. Castling on the kingside
is sometimes called castling short and castling on the queenside
is called castling long; the difference being based on whether the
rook moves a short distance (two squares) or a long distance (three
Castling is in most European languages other than English known
as 'rochieren / rochada / roque' or some other derivative of the
same Persian root from which also the English word 'rook' is derived,
while the local adjectives meaning 'long' and 'short' are used in
those countries to refer to 'queenside/kingside castling'. Castling
was added to European chess in the 14th or 15th century and did
not develop into its present form until the 17th century. Asian
versions of chess do not have such a move.
Castling is permissible only if all of the following conditions
- The king has not previously moved;
- The chosen rook has not previously moved;
- There must be no pieces between the king and the chosen rook;
- The king is not currently in check.
- The king must not pass through a square that is under attack
by enemy pieces.
- The king must not end up in check
- The king and the chosen rook must be on the same rank. (This
rule was added to allow for the possibility of a pawn being promoted
to a rook, which then castles!)
Castling is an important goal in the early part of a game. It serves
two purposes: it moves the king into a safer position away from
the centre of the board, and it moves the castle to a more active
position in the centre of the board (it is even possible to checkmate
|Castling on the King's Side
The choice as to which side to castle depends on an assessment
of the trade-off between king safety and activity of the rook. Kingside
castling is generally safer, because the king ends up closer to
the edge of the board and all the pawns on the castled side are
defended by the king. In queenside castling, the king is placed
closer to the centre and the pawn on the a-file is undefended; the
king is thus often moved to the b-file to defend the a-pawn and
to move the king away from the centre of the board. In addition,
queenside castling requires moving the queen first; therefore, it
may take slightly longer to achieve than kingside castling. Queenside
castling places the rook more effectively. It is often immediately
active, whereas with kingside castling a tempo may be required to
move the rook to a more effective square.
It is common for both players to castle kingside, and rare for
both players to castle queenside. Castling on opposite sides usually
results in a fierce fight as both players' pawns are free to advance
to attack the opposing king's castled position without exposing
the player's own castled king.
|Castling on the Queen's Side
If the king is forced to move before it has the opportunity to
castle, the player may still wish to manoeuvre the king towards
the edge of the board and the corresponding rook towards the centre.
When a player takes three or four moves to accomplish what castling
would have accomplished in one move, it is sometimes called artificial
castling, or castling by hand.
Under the strict touch-move rules enforced in most tournaments,
castling is considered a king move. A player who intends to castle
but touches the rook first would be committed to make a rook move,
and thus will not be permitted to castle. The correct way to castle
is to first move the king. As usual, the player's mind may change
between all legal destination squares for the king until it is released.
When the two-square king move is completed however, the player has
formally chosen to castle, and the rook must be moved accordingly.
A player who performs a forbidden castling must return the king
and the rook to their original places and then move the king, if
there is another legal king move, including castling on the other
side. If there is no legal king move, the touch-move rule does not
apply to the rook. The official rules require that the entire move
be completed using only a single hand. Neither of these two rules
is commonly enforced in casual play, nor commonly known by non-competitive
The right to castle must be the same in all three positions for
a valid draw claim under the threefold repetition rule.