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Chess - Rules

# SYNOPSIS

The rules of chess, including a glossary of terms.

# DESCRIPTION

This document describes the rules of chess, along with associated terminology that is used in the description and implementation of the Games::Chess::Referee Perl module.

# EQUIPMENT

The equipment of chess consists of a board, two players, and a set of chessmen.

## The Board

The board consists of a square lattice of equally sized square spaces, with no gaps in between the spaces. Sometimes the spaces are called squares.

The figure below shows a common way to display the board in a text-only situation. The dots on the interior represent the spaces.

```        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```

The spaces on the board are of two different kinds. Regardless of the actual distinguishing characteristics involved, one kind is said to be of the color `white', and the other is `black'. These names are used to correspond to the names given to the players (see below).

The rows are called ranks, and the columns are called files.

In algebraic notation, the ranks are given numbers from 1 to 8 and the files and are given letters from `a' to `h'. The board is conventionally drawn as above, with the number 1 below the number 8 and the letter `a' to the left of the letter `h'. Each space has a name formed by naming its file letter, followed by its rank number. So, the four corners of the board are named: a1, a8, h1, and h8.

In traditional notation, the ranks are divided into two groups, the queen's side is the left half of the figure above, and the king's side is the right half. The ranks are numbered as in algebraic notation, and the files are named, from left to right in the figure above, queen's rook, queen's knight, queen's bishop, queen, king, king's bishop, king's knight, and king's rook.

The lower-left space (a1, or queen's rook 1) is black, and neighboring spaces along ranks and along files alternate between white and black, so that no two spaces sharing an edge are of the same color. Another way to remember this is the mnemonic ``light on right'', which has an equivalent effect.

## The Players

Chess is played by two players. One player is designated color `white', and the other is designated `black'. White's chessmen are designated by upper-case letters, while black's are designated by lower-case letters.

## The Chessmen

Each player has a collection of objects corresponding to its color, as follows:

• Eight pawns, designated by the letter `p'.
• Two rooks, designated by the letter `r'.
• Two knights, designated by the letter `n'.
• Two bishops, designated by the letter `b'.
• One queen, designated by the letter `q'.
• One king, designated by the letter `k'.

# PLAY

This section describes the play of the game.

## The Object

The object of Chess is to trap the opponent's king.

## The Setup

The initial board setup is as follows:

```        a b c d e f g h
+-----------------+
8 | r n b q k b n r | 8
7 | p p p p p p p p | 7
6 | . . . . . . . . | 6
5 | . . . . . . . . | 5
4 | . . . . . . . . | 4
3 | . . . . . . . . | 3
2 | P P P P P P P P | 2
1 | R N B Q K B N R | 1
+-----------------+
a b c d e f g h```

An easy way to remember how to set up the kings and queens correctly is to place the queen on the space of its same color, then place the king next to it.

One player is be designated `white' and the other `black'. White always plays first, so it is customary to switch colors each game when the same players will play multiple games. Choosing the player to be white for the first game is either by mutual agreement, or random choice. One way is for one player to fetch a pawn of each color, mixing them up so that one is in each hand and then allow the opponent to choose one hand.

## The Procedure

Gameplay proceeds by the players alternately executing plies. A ply is the taking by a player of a legal action. A white ply followed by a black ply in response is called a move (equivalent to a turn in other games). A ply consists of moving a piece to a new space according to the movement style of the piece, or capturing an enemy piece, or one of a number of special moves.

# MOVES AND CAPTURES

The chess pieces capture in the same ways they move. But, pawns move and capture in distinct ways. In no case may two pieces occupy the same space. When one player's piece lands on a square containing one of the opponent's pieces, the opponent's piece is captured and removed from the board. It is illegal to land a piece on a square already occupied by a piece of one's own color.

## Pawns

A pawn may move one square towards the opponent's side of the board, as long as the destination space is not occupied. If the pawn has not moved previously, and so is on its home square at the beginning of the turn, the player may choose to move it forward two squares, provided that both of the squares in question are unoccupied.

```        a b c d e f g h
+-----------------+
8 | . . . . . . . . | 8
7 | . . . . . . . . | 7
6 | . . . . . . . . | 6
5 | . . . . . . . . | 5
4 | . . . . . # . . | 4
3 | . . . . . # . . | 3
2 | . . . . . P . . | 2
1 | . . . . . . . . | 1
+-----------------+
a b c d e f g h```

A pawn may capture an enemy piece by moving diagonally forward so that the result is as if it had moved one square forward, turned left or right, and moved one space. For this to be legal, the destination space must contain an enemy piece. It does not matter if the space just forward of its initial position is occupied or not.

```        a b c d e f g h
+-----------------+
8 | . . . . . . . . | 8
7 | . . . . . . . . | 7
6 | . . . . . . . . | 6
5 | . . . # . # . . | 5
4 | . . . . P . . . | 4
3 | . . . . . . . . | 3
2 | . . . . . . . . | 2
1 | . . . . . . . . | 1
+-----------------+
a b c d e f g h```

If the player has a pawn on the rank two away from the opponnent's original pawn rank (rank 5 for white, rank 4 for black), a special rule may apply. If the ply just prior to this one consisted of the opponent moving one of its pawns from the home rank forward two squares in such a way that it would have been vulnerable to a diagonal capture had it moved only one square, then on this very next ply, the player may capture the enemy pawn by making the diagonal move onto the empty space, as if the enemy pawn had been captured ``in passing'' (officially, en passant).

```        a b c d e f g h
+-----------------+
8 | . . . . . . . . | 8
7 | . . . . . . . . | 7
6 | . . . . # . . . | 6
5 | . . . . p P . . | 5
4 | . . . . . . . . | 4
3 | . . . . . . . . | 3
2 | . . . . . . . . | 2
1 | . . . . . . . . | 1
+-----------------+
a b c d e f g h```

If a pawn lands on the opponent's home rank, it is promoted. The player may choose any piece other than a pawn or a king to replace the pawn. The player need not have previously lost the piece to a capture, so it is possible to have a board position with more than one queen or more than two of some other piece type.

```        a b c d e f g h
+-----------------+
8 | . . . . P . . . | 8
7 | . . . . . . . . | 7
6 | . . . . . . . . | 6
5 | . . . . . . . . | 5
4 | . . . . . . . . | 4
3 | . . . . . . . . | 3
2 | . . . . . . . . | 2
1 | . . . . . . . . | 1
+-----------------+
a b c d e f g h```

## Rooks

Rooks move in straight lines along ranks and along files. Each rook move consists of a move in a single direction of one or more spaces. A rook may not pass through an occupied to space to get to its destination.

```        a b c d e f g h
+-----------------+
8 | . . # . . . . . | 8
7 | . . # . . . . . | 7
6 | . . # . . . . . | 6
5 | # # R # # # # # | 5
4 | . . # . . . . . | 4
3 | . . # . . . . . | 3
2 | . . # . . . . . | 2
1 | . . # . . . . . | 1
+-----------------+
a b c d e f g h```

## Knights

Knights move by hopping over spaces, so there is no need for the spaces between it starting and ending positions to be empty. They are the only pieces with this capability. A knight moves by jumping in an `L' shape. First, choose a directioon: left, right, forward, or backward. Count two squares in this direction, then count one square in either of the perpendicular directions. This is the destination space. The destination space will always be of opposite color to the starting space.

For the knight marked `N' in the figure below, the eight possible destination spaces are marked with `#'.

```        a b c d e f g h
+-----------------+
8 | . . . . . . . . | 8
7 | . . . . . . . . | 7
6 | . . # . # . . . | 6
5 | . # . . . # . . | 5
4 | . . . N . . . . | 4
3 | . # . . . # . . | 3
2 | . . # . # . . . | 2
1 | . . . . . . . . | 1
+-----------------+
a b c d e f g h```

## Bishops

Bishops move in diagonal paths, so that they always remain on spaces of the same color as their starting space. Choose to move forward or backward, and choose a number of spaces. Move the same number of spaces also either left or right.

```        a b c d e f g h
+-----------------+
8 | . . . . . . . # | 8
7 | # . . . . . # . | 7
6 | . # . . . # . . | 6
5 | . . # . # . . . | 5
4 | . . . B . . . . | 4
3 | . . # . # . . . | 3
2 | . # . . . # . . | 2
1 | # . . . . . # . | 1
+-----------------+
a b c d e f g h```

## The Queen

The queen moves and captures like a combination of a rook and a bishop. Each ply, the queen may be moved either like a rook or like a bishop.

```        a b c d e f g h
+-----------------+
8 | . . . # . . . # | 8
7 | # . . # . . # . | 7
6 | . # . # . # . . | 6
5 | . . # # # . . . | 5
4 | # # # Q # # # # | 4
3 | . . # # # . . . | 3
2 | . # . # . # . . | 2
1 | # . . # . . # . | 1
+-----------------+
a b c d e f g h```

## The King

The king moves like the queen, except that it can only move one space at a time.

```        a b c d e f g h
+-----------------+
8 | . . . . . . . . | 8
7 | . . . . . . . . | 7
6 | . . . . . . . . | 6
5 | . . # # # . . . | 5
4 | . . # K # . . . | 4
3 | . . # # # . . . | 3
2 | . . . . . . . . | 2
1 | . . . . . . . . | 1
+-----------------+
a b c d e f g h```

There are two special moves available to the king, called castling. If the king has not moved from its home space, and one of the rooks has not moved from its home square, castling on that side may be possible. Provided that the spaces between the king and the rook in question are empty, and that none of the spaces, including the king's and the rook's current locations, are guarded by enemy pieces, th king is permitted to move two spaces in the direction of the rook, and the rook `simultaneously' (in the same ply) takes up the space between the king's old and new locations (`hopping' over the king).

The figure below shows the setup before and after castling short, or castling kingside.

```        a b c d e f g h             a b c d e f g h
+-----------------+         +-----------------+
8 | . . . . . . . . | 8     8 | . . . . . . . . | 8
7 | . . . . . . . . | 7     7 | . . . . . . . . | 7
6 | . . . . . . . . | 6     6 | . . . . . . . . | 6
5 | . . . . . . . . | 5     5 | . . . . . . . . | 5
4 | . . . . . . . . | 4     4 | . . . . . . . . | 4
3 | . . . . . . . . | 3     3 | . . . . . . . . | 3
2 | . . . . . . . . | 2     2 | . . . . . . . . | 2
1 | . . . . K . . R | 1     1 | . . . . . R K . | 1
+-----------------+         +-----------------+
a b c d e f g h             a b c d e f g h    ```

The figure below shows the setup before and after castling long, or castling queenside.

```        a b c d e f g h             a b c d e f g h
+-----------------+         +-----------------+
8 | . . . . . . . . | 8     8 | . . . . . . . . | 8
7 | . . . . . . . . | 7     7 | . . . . . . . . | 7
6 | . . . . . . . . | 6     6 | . . . . . . . . | 6
5 | . . . . . . . . | 5     5 | . . . . . . . . | 5
4 | . . . . . . . . | 4     4 | . . . . . . . . | 4
3 | . . . . . . . . | 3     3 | . . . . . . . . | 3
2 | . . . . . . . . | 2     2 | . . . . . . . . | 2
1 | R . . . K . . . | 1     1 | . . K R . . . . | 1
+-----------------+         +-----------------+
a b c d e f g h             a b c d e f g h    ```

## Global Considerations

Chess is an idealization of a battle. Each of the pieces has distinct capabilities that must be accounted for in formulating strategies and tactics. When examining a board position and selecting a move, it is important to pay attention to the guards and threats present before and resulting from the move. A piece guards a square if the piece could capture an enemy piece on that square on the next ply, not accounting for the fact that the enemy piece may not be able to actually execute the capture due to constraints on its own king's disposition.

It is illegal to execute a ply that leaves the king on a guarded square. A player that manages to execute a legal ply that causes the opposing king to be on a guarded square has put the other player in `check'.

For example, in this board position:

```        a b c d e f g h
+-----------------+
8 | k . . Q . . . R | 8
7 | . . . . . . . . | 7
6 | . . . . . . . . | 6
5 | . . . . . . . . | 5
4 | . . . . . . . . | 4
3 | . . . . . . . . | 3
2 | . . . . . . . . | 2
1 | B . . . K . . . | 1
+-----------------+
a b c d e f g h```

the move 1. Ke1-d1 is not legal, despite the fact that the response 1. ... Qd8xd1 is not credible in the sense that the queen is pinned at d8 by the white rook on h8. It is sufficient that the queen is guarding the square d1 for it to be illegal for the king to move there.

There are three ways to get one's king out of check:

1. Capture the piece that guards the king's space. If it happens that there is more than one such piece (called double check), this option is not available.
2. Interpose a piece. If the opponent's piece is on a neighboring square, this option is not available.
3. Move the king to an empty unguarded square.

Since it is not permitted to have one's king on a guarded square, the other player must respond by securing its king on its next ply, regardless of any other opportunites there may be on the board. If there are no legal plies available that accomplish this, the player has lost, and the situation is called `checkmate'. If a player is not in check, but still has no legal plies, then it is `stalemate', which is the Chess term for a tie game.

The king may never be exposed to the gaze of another piece. So, each potential move must be screened first to make sure that after it is executed, no enemy piece guards the king's square. Even if it would be illegal for the other piece to move away from its present location on the next ply (meaning that although the piece guards the square, it does not threaten it)

## Move Notation

Each move is given its own line of text. First, the move number, then white's ply, then black's response. A ply is denoted by a piece type, followed by a starting square name, an action type, an ending square name, and a qualifier. The piece types are: `P` for pawn (usually omitted by convention), `R` for rook, `N` for knight, `B` for bishop, `Q` for queen, and `K` for king. Square names follow algebraic notation, actions are occupy (`-`) and capture (`x`). The primary qualifiers are: `+` for check, `#` for checkmate, `!` for good move, `?` for bad move, and `??` for blunder. In practice some of these elements can be omitted without ambiguity, and often are.

The game below is an example of scholar's mate.

```    1. e4     e5
2. Bc4    h6?
3. Qh5    ...    Qf3 would also serve.
3. ...    a5??
4. Qxf7#```

Note how move 3 has been split into two lines, and in each case the elided ply is replaced with ellipses. This is a common form when providing commentary on the progress of a game.

# IMPLEMENTATION

This file describes a notation for describing the possibilities and effects of plies in chess- like games.

The king may not move into check, where check is not defined as the ability of the enemy piece to execute a capture (since this would involve checking to see that it was legal to vacate the space it was on, which would introduce recursion), but on the ability of the enemy piece to `see' the king.

## Piece Predicates

These predicates refer to the ply piece by default:

`white`

The piece under consideration is white.

`black`

The piece under consideration is black.

`home`

The piece is in its home position and has not moved prior.

## Space Predicates

These predicates refer to the `to' square by default:

`empty`

The resulting space is not occupied.

`open`

The spaces between the starting and resulting square are empty.

`clear`

The intervening spaces are open and not guarded, and the starting and ending spaces are not guarded. This is used in the description of castling.

`enemy`

The space is occupied by an enemy piece.

`friend`

The space is occupied by a friendly piece.

`pawn`

The space contains a pawn.

`guarded`

The square is guarded by an enemy piece.

## Other predicates

`last_from`

True if the square given is the `from' from the last ply.

`last_to`

True if the square given is the `to' from the last ply.

`last`

True if the first square is the `from' and the second square is the `to' of the last ply.

## Relative Spaces

`left`

The space immediately to the left, delta of [-1, 0].

`right`

The space immediately to the right, delta of [+1, 0].

`kings_rook`

The rook home position on the king's side.

`queens_rook`

The rook home position on the queen's side.

## Vectors

vectors may be added, subtracted, and multiplied by a constant.

`ahead`

The space immediately `ahead', meaning away from the home row of the color.

`king`

Towards the king one square.

`queen`

Towards the queen one square.

## Compound predicates

`white_passant`

` (enemy(left) and pawn(left) and last(left + [0, 2], left)) or (enemy(right) and pawn(right) and last(right + [0, 2], right)) `

`black_passant`

` (enemy(left) and pawn(left) and last(left - [0, 2], left)) or (enemy(right) and pawn(right) and last(right - [0, 2], right)) `

`passant`

` (white and white_passant) or (black and black_passant) `

The default action is for the piece to move to the destination and the enemy piece at that location to be removed. For specialty moves, these can be overridden.

It is assumed that any moves generated by these rules that produce off-board result positions are rejected.

## Actions

`mutate`

After the move, mutat to the designated piece (used for pawn promotion in standard chess).

## Formalized Plies

Pawns:

```    [ahead]         if empty;
[ahead * 2]     if empty and clear and home;
[ahead + left]  if enemy or passant;
[ahead + right] if enemy or passant.```

Rooks:

```    [0, ?]   if clear and not friend;
[?, 0]   if clear and not friend.```

Knights:

```    [+1, +2] if not friend;
[+1, -2] if not friend;
[-1, +2] if not friend;
[-1, -2] if not friend;
[+2, +1] if not friend;
[+2, -1] if not friend;
[-2, +1] if not friend;
[-2, -1] if not friend.```

Bishops:

```    [?,  ?]  if clear and not friend;
[?, -?]  if clear and not friend.```

Queens:

```    [0, ?]   if clear and not friend;
[?, 0]   if clear and not friend.
[?, ?]   if clear and not friend;
[?, -?]  if clear and not friend.```

Kings:

```    [-1, -1] if not friend and not threat;
[-1,  0] if not friend and not threat;
[-1, +1] if not friend and not threat;
[ 0, -1] if not friend and not threat;
[ 0, +1] if not friend and not threat;
[+1, -1] if not friend and not threat;
[+1,  0] if not friend and not threat;
[+1, +1] if not friend and not threat;
[king * 2]  if home and home[kings_rook] and clear[king, kings_rook];
[queen * 3] if home and home[queens_rook] and clear[king, queens_rook];```

We still need notations for the rook movements during castling.

Globally: `not threat[king]`;

# AUTHOR

Gregor N. Purdy (`gregor@focusresearch.com`).