Definition:Chess/Rules

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Definition

With the exception of the knight, no chess piece may move past a square which is occupied by another piece.

Under Attack

If a chess piece $A$ may legally move to a square which is occupied by a piece $B$ of the opposite colour, $B$ is said to be under attack by $A$.

In the same way, any square to which $A$ may legally move may also be said to be under attack.

Capture

If, during the course of its move, a chess piece is able to move into a square which is occupied by a piece of the opposite colour, it may do so.

Having done so, the piece of the opposite colour is then removed from the board and takes no further part in the game.

This is called capture.

Note that the behaviour of pawns is different.

Pawn

A capture in chess by a pawn works completely differently from that by any other piece.

It may happen in one of $2$ circumstances.

Normal Pawn Capture

If one of the $2$ diagonally adjacent squares towards the opposing player is occupied by an opposing piece, the pawn may move into that square and capture that piece.

En Passant Pawn Capture

Let pawn $a$ be on the player's $5$th rank.

Let pawn $b$ be of the opposite colour to pawn $a$.

Let pawn $b$ be on its starting position, on one of the files adjacent to the one occupied by pawn $a$.

Let pawn $b$ move forward $2$ spaces, in the process crossing over one of the squares which is under attack from pawn $a$.

Then pawn $a$, on its next move only, may move into that square crossed over by pawn $b$, and capture pawn $b$ "while it is passing".

This mode of capture is known as capture en passant.

Castling

Castling is a special move where, in one move, the king moves two squares towards a rook, and the rook jumps over the king and lands on the next square the other side of the king.

This can only be done if:

$(1): \quad$ The king and rook have not been moved yet.
$(2): \quad$ The squares in between the king and rook are empty.
$(3): \quad$ The king is not in check, and neither of the $2$ squares it moves through are under attack.

In Check

Uniquely among the chess pieces the king may not legally move into a square which is under attack by a piece of the opposite colour.

That is, a king may not be moved so as to be placed under attack.

If player $a$ moves a piece so as to place the king of player $b$ under attack, then player $b$ is said to be in check.

Player $a$ is then obliged to say "Check."

Player $b$ must make a move so that his or her king is no longer under attack.

Such a move by player $b$ is known as getting out of check.

Checkmate

If player $a$ has put player $b$'s king in check, but player $b$ has no legal move that will get him or her out of check, then player $a$ has won.

This situation is known as checkmate.

Player $a$ is said to have checkmated player $b$.