# Definition:Chess

## Definition

Chess is a game for two players.

It is played using pieces placed on a chessboard, where the players take turns to move a piece.

A piece is captured if an opposing piece moves onto the same space.

The opposing piece is attacking.

The captured piece is then removed from the board, unless it is a king.

It is forbidden to capture your own piece.

A piece is threatened if and only if it could be captured in one move by any piece.

A piece cannot move over another piece, unless it is a knight, or it is castling.

If a king is captured, the player who captured it wins immediately.

A king is put in check if and only if it is threatened by any piece.

This is checkmate if and only if the player who owns the king cannot prevent the king from being captured using one move.

The only other way to win is if a player indicates that he resigns, traditionally by knocking over his own king.

### Move

Each turn in a game of chess consists of transferring a piece from one square to another, according to the rules.

Such a transfer is known as a move.

There is one exception: the operation of castling involves the moving of $2$ pieces.

### Position

The specific placement of all the chess pieces on a chessboard after a given move is known as the position.

## Chess Player

The two players in a game of chess are identified by the colour of the pieces they are using.

The sets of pieces are made from materials of two contrasting colours, conventionally referred to as black and white.

Most everyday chess sets actually are black and white.

### White

The chess player who is using the lighter-coloured pieces is conventionally referred to as white.

### Black

The chess player who is using the darker-coloured pieces is conventionally referred to as black.

A diagram of a chessboard is conventionally positioned on a page such that white is at the bottom, playing upwards towards black, and black is at the top, playing downwards towards white.

## Chess Set

The set of all the pieces that are used by the chess players is known as a chess set.

## Chessboard

A chessboard is an array (usually square) of alternating dark and light squares (conventionally referred to as black and white, even though they may well not be those actual colours).

 a b c d e f g h 8 8 7 7 6 6 5 5 4 4 3 3 2 2 1 1 a b c d e f g h

This may be referred to as an $m \times n$ chessboard, where:

$m$ is the number of ranks
$n$ is the number of files.

For the conventional chessboard, as depicted above, $m = n = 8$.

### Rank

A row of a chessboard is known as a rank.

It is identified by number:

$1$ is the rank nearest white
The ranks are numbered sequentially to the rank nearest black.

### File

A column of a chessboard is known as a file.

It is identified by letter:

$\text a$ is the file on the edge of the board to the left of white (that is, to the right of black)
The files are lettered sequentially up to the file on the edge of the board to the left of black (that is, to the right of white).

### Square

The individual spaces of the chessboard are referred to as its squares.

They are conventionally identified by a letter-number pair, where:

the letter identifies its file
the number identifies its rank.

## Pieces

There are a number of different kinds of chess piece.

They are distinguished by their modes of movement, as follows:

### King

The king can be moved one square in any direction, including diagonally.

It cannot be moved into a position where it is in check.

### Queen

The queen can be moved any number of squares in any of $8$ directions: forwards, backwards, left, right, and the $4$ diagonals.

It cannot be moved past a square occupied by another piece.

### Bishop

The bishop can be moved any number of squares in any of the $4$ diagonal directions.

It cannot be moved past a square occupied by another piece.

### Knight

The knight can be moved two squares in any orthogonal direction (up, down, left or right), and then one square at right angles to that direction.

It passes over intervening pieces and does not stop until reaching the end of its move.

It cannot be moved onto a square occupied by another piece of the same colour.

### Rook

The rook can be moved any number of squares in any of the $4$ orthogonal directions: up, down, left or right.

It cannot be moved past a square occupied by another piece.

### Pawn

A pawn may move in the following modes.

$(1): \quad$ It may move one square towards the opposing player, if and only if that square is empty.

$(2): \quad$ If it has not yet been moved, and if and only if both of the squares are empty, it may move $2$ squares forward instead of $1$.

Some authorities do not classify the pawn as a piece.

## Starting Position

The starting position of a game of chess is:

 a b c d e f g h 8 8 7 7 6 6 5 5 4 4 3 3 2 2 1 1 a b c d e f g h

Note the following:

$(1): \quad$ The top left square is white
$(2): \quad$ The queens start on squares of their own colours
$(3): \quad$ White is always first to move.

## Rules

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 $b$.

## Draw

A game of chess may end in a draw in several possible ways:

### Agreement

When both players agree that neither can win, a game of chess can be declared drawn.

### Stalemate

Stalemate is a situation where neither player is in check, but the player whose turn it is to move cannot do so legally.

The game of chess is drawn.

### Repetition

If the same position for all pieces occurs multiple times throughout a game of chess, this is a draw.

The exact number of positions is dependent on the specific rules used, but $3$ is conventional.

### Fifty Move Rule

If, during $50$ moves, no pawn has been moved and no capture has been made, either player may claim a draw.

This rule is not always used.

### Insufficient Pieces

If neither player has sufficient pieces to force checkmate, then the game of chess is drawn.

## Also see

• Results about chess can be found here.