# Rules of Chess

Ole K. Christensen

# 1. Introduction

If you are not yet familiar with the rules of chess, or you are just starting to play, you can read the following tutorial. We will also touch briefly on the basic usage of Sigma Chess when needed.

Chess is a battle between two players called White and Black. The game takes place on an 8x8 square board with 16 white and 16 black pieces, which are initially placed as shown in the diagram.

White to move

White is leading (moving) the white pieces, and Black is leading the black pieces. White always makes the first move (i.e. moves one of his own pieces). Then Black makes a move, after which it's White's turn again e.t.c.

A piece can move to an empty square but not to a square that is already occupied by another piece of the same colour. If a piece moves to a square that is occupied by an opponent piece, the opponent piece is said to be "captured" and must be removed from the board. Thus there can never be more than one piece on a square.

- o O o -

This tutorial is a "game collection" designed as an online chess book. It is divided into a number of "sections", each of which either contains text or an annotated chess game. Once you have completed reading a section, you can move on to the next section by pressing Cmd-right arrow (which is a shortcut to the "Next Game" command in the "Collection" menu). If a section contains moves (i.e. if the VCR style buttons > and >> are enabled), you should replay these moves first before moving on to the next section.

The text "[DIAGRAM]" on a single line indicates that a diagram will be included of the current board position when printing.

The collection windowshows a "table of contents" of this online chess book showing the titles of each section. You can move directly to a section by double clicking on the title in the list.

Press Cmd-right arrow to continue.

## The Pieces

Before we can go into the details of how each piece actually moves, we first need to get acquainted with the various piece types: The white piece in the lower lefthand corner resembling a tower is called a "rook". Next to the rook we find the horselike "knight", followed by the "bishop", the "queen" and the "king". Next to the king we then have one more bishop, knight and rook. In front of these 8 pieces on the second row are the 8 "pawns". Black is equipped with a similar set of pieces.

To summarize there are thus 6 different piece types, and each player is initially in possession of 16 pieces:

1 King
1 Queen
2 Rooks
2 Knights
2 Bishops
8 Pawns

Each of these 6 piece types are identified solely by the way they move. For instance, the fact that the queen is stronger than the rook is not because the queen as such has higher rank than the rook (as in most card games and board games such as Stratego). Rather, the queen moves in a way superior to the rook.

## Objective of Chess

The objective of the game is to conquer the opponent king. The king is thus the most important piece in chess, although not the strongest in terms of the ability to capture opponent pieces. This actually is, as we shall soon see, the queen.

The side to move is said to be in "check" if his king is attacked by an opponent piece, i.e. if the king could be captured immediately by that piece if it was the opponent's turn to move. It is illegal for the side to move to leave his king in check or to make a move that will allow the opponent to capture his king immediately thereafter.
If the side to move is check and he has no legal moves (i.e. no matter what he does, the opponent can capture his king on the next move), then he is said to be "check mate" or just "mate" and has lost the game. The objective of a chess game can now be stated more precisely: To check mate the opponent king.

## Chess Move Notation

Various forms of chess notation exist for describing the moves of a game. The most commonly used form is the so-called algebraic notation, where each square of the board is identified by its "file" (column) and "rank" (row). The files are described from left to right by the letters 'a' to 'h', whereas the ranks are numbered from 1 to 8 as shown on the diagram.

In the initial position, the white pieces thus occupy ranks 1 and 2. For instance, the white king is located at the square e1, the black queen at d8, and the four central squares are named d4, d5, e4 and e5.

The notation for a single chess move basically consists of an indication of the origin square followed by the destination square. If the move is not a pawn move, the origin square is prefixed with a piece letter (K = King, Q = Queen, R = Rook, B = Bishop, N = knight) indicating the moving piece. When using figurine notation, the piece letter is replaced by a miniature picture of the moving piece.

For instance, the move Ng1-f3 indicates a knight moving from the square g1 to f3, whereas the move d2-d4 indicates a pawn moving from d2 to d4. For capturing moves the hyphen '-' is replaced with a 'x' (e.g. Rc4xc8 for a rook on c4 capturing a piece on c8). There are a few exceptions to these rules which will be described later.

Two forms of algebraic notation exist: "long" and "short" notation. In the long algebraic notation (LAN) we indicate both the source and destination square (e.g. Ng1-f3, Rc4xc8 and d2-d4). In the short algebraic notation (SAN) the source square is omitted (e.g. Nf3, Rxc8 and d4) unless this makes the move ambiguous, i.e. if more than one piece of the same type can move to the same square.

For the time being we will stick with the long notation because it is simpler although somewhat bulky. We will return to the more difficult but more compact and economic short notation later. We suggest that you select long notation from the "Notation" submenu in the "Display" menu before continuing.

# 2. Piece Movement

Now that the various piece types have been introduced we are now in a position to describe the rules governing the movement of each piece. We shall start with the kings followed by the rooks, bishops, queens, knights and pawns. In each case we shall illustrate the piece movement through brief game samples.

NOTE: To see the legal moves for any piece belonging to the side to move, you can hold down the option key and click on the desired piece. This will indicate all legal destination squares for that piece with a blue frame. It is recommended to use this facility often in the following discussion.

## The King

The diagram on the left shows a position with a white king and rook versus a lonely black king where White is to move.

White to move

As we will learn later, this is an easy win for White, but for the moment let us forget about the white rook and concentrate on the kings.

A king can move 1 square in any of the 8 directions: up, down, left, right and diagonally (hold down the option key and click on the white king to see his moves).

Now, assume that white decides to move his king right to the square e4 (select the "Redo Move" command from the game menu):
1 Ke4 . . .
It is now Black's turn to move. Since his king is on the edge of the board, he only has 5 available squares to go to.

As we will see later, the Black king can in this piece configuration only be captured on the edge of the board. To postpone his defeat he therefore decides to move away from the edge to the square g6.
1 . . . Kg6
It is now White's turn again, and we now encounter an important exception to the rule, that the king can move to any of the 8 squares surrounding him. It is namely illegal for the white King to move to the square f5, as this would enable the black King to capture the white King immediately afterwards. Thus there must always be at least one square between the two kings.
2 Ke5 . . .
White instead moves his king to e5 to keep the Black king from escaping too far from the edge. Now it is Black's turn...
2 . . . Kf7
This concludes the movement of kings for the moment. We will now introduce the rook.

## The Rook

The rook can move both vertically and horisontally but not diagonally. Contrary to the King, the rook can move more than one square at a time, but it cannot pass through occupied squares (hold down the option key and click on the white rook in the diagram to see possible moves):

White to move

3 Rg1 . . .
This move cuts off the black king from going to the g file. As before, it is illegal for a king to expose himself to attack. If Black for instance moved to g6, the White rook could capture him by playing Rg1xg6.
3 . . . Ke7
4 Rg7+ . . .
This move directly threatens the Black king, i.e. Black is in check which is indicated by a "+" sign after the move.

Now Black is forced to the edge of the board, since the white king controls d6, e6 and f6 and since the white rook controls f7 and d7 (and in fact the whole 7th rank).
4 . . . Ke8
5 Kd6 Kd8
6 Rg8#
Black has now lost the game since his king is check mate (indicated by a "#" sign), i.e. Black is check and he has no legal moves, because no matter where his king goes, White can capture him on his next move (of course 5. ... Ke8-d8 is not Black's best move. The move 5. ... Ke8-f8 still loses but postpones the mate: 6. Rg7-e7 Kf8-g8 7. Kd6-e6 Kg8-f8 8. Ke6-f6 Kf8-g8 9. Kf6- g6 Kg8-f8 10. Re7-e6 Kf8-g8 11. Re6- e8# mate).

## The Bishop

The bishop is similar to the rook, the difference being that bishops only move diagonally (hold down the option key and click on one of the bishops on the board to see their moves). Note that the bishop on e3 cannot move to b6 or a7 in the diagram because its path is blocked by the white king:

White to move

A bishop thus always moves on identically coloured squares, and is therefore weaker than a rook that can control a whole file or rank. A single bishop and a king also cannot force mate no matter how the lonely king is placed on the board.

Two bishops as in the diagram are sufficient though to force mate (provided they are moving on different coloured squares).
1 Bd4 . . .
This move illustrates the power of two adjacent bishops. They are blocking the right side of the board.
1 . . . Ke7
2 Bd5 . . .
Again "tightening" the grip by preventing the black king from escaping.
2 . . . Kd7
3 Bf6 Kc7
4 Bc6 . . .
Now the black king is forced to retreat to the edge of the board, which - as in the case of the rook - is the only place where White can force mate with two bishops.
4 . . . Kc8
5 Kb6 Kb8
6 Bd7 Ka8
7 Bg5 . . .
An important waiting move. The more obvious move Bf6-e5 which takes away the last free square from the black king actually leads to a draw because of a special rule: Although the black king has no legal moves after Bf6-e5, he is not check mate because he is not check. Instead black is said to be "stale mate", which by definition is a draw.
7 . . . Kb8
8 Bf4+ Ka8
This is Black's only move, and now White can force mate in the next move:
9 Bc6#

## The Queen

The queen combines the movement of the rook and bishop, and consequently makes the queen stronger than both. Forcing mate with the queen is a relatively simple task:

White to move

1 Qc7+ . . .
Forces the black king to the back rank.
1 . . . Kf8
2 Kf6 Kg8
3 Qg7#
Mate!

## The Knight

The knight moves in a special manner, which does not resemble that of the queen, rook and bishop. The knight moves 1 square in one direction followed by 2 squares perpendicular to that direction. The knight on d3 in the diagram can thus move to the squares c1, e1, b2, f2, b4, f4, c5 and e5:

White to move

Hold down the option key and click on the knight to see the moves.

Unlike the queen, rook and bishop the knight can pass (or jump) over occupied squares.

A single knight cannot force mate no matter how the lonely king is placed. It is possible to check mate a lonely king with two knights, but not if the lonely king "plays" optimally.
1 Ne5+ Ke7
2 Nc6+ Kd7
3 Nxe8 Kxe8

## The Pawn

The pawn is the weakest of all pieces. The movement is defined by the following rules:

1) If the square immediately in front of the pawn is empty, the pawn can move to that square.

2) If the pawn is located on the second rank (and hence has never been moved) it may move two squares forward if both squares are empty.

3) If the square left or right to the square in front of the pawn is occupied by an enemy piece, the pawn may capture that piece.

4) A pawn that advances to the 8th rank is "promoted" to either a queen, rook, bishop or knight. The choice is up to the player, but normally a queen is chosen because it is the strongest of the pieces. The notation for promotion moves consists of appending an "=" sign followed by a piece letter indicating the selected piece (e.g. g7-g8=Q for a queen promotion).

Thus a pawn can never move backwards but only forward towards the opponent side of the board.

White to move

The pawn on c2 can move to c3, but also to c4 because it has not been moved before. The pawns on b5 and f4 cannot move. They are blocked by black pieces.

The pawn on h3 can only move to h4, whereas the pawn on e5 can move to e6 as well as capture the black pawn on d6.

The pawn on g7 can only move to g8 and must be promoted to a queen, rook, knight or bishop (hold down the option key and click on the various white pawns):
1 g8=Q dxe5
2 fxe5 Kxe5

# 3. Special Rules

There are a few special rules in addition to the already described movement rules:

1) Castling
2) En passant
3) Draw by repetition
4) The 50 move rule
5) Draw Offers

## Castling

"Castling" is a special move that involves both a king and a rook. In the diagram white can castle "king sides" by moving his king to g1 and the rook on h1 to f1. He also has the option of castling "queen sides" by moving his king to c1 and the rook on a1 to d1:

White to move

Note, that castling is considered a single move even though it involves two pieces of the same colour. The move notation for king sides castling is "O-O" and for queen sides castling it is "O-O-O".

Castling is only legal if the following conditions are satisfied:

1) Neither the king nor the rook in question may have moved before.
2) The squares between the king and rook must be empty.
3) The king may not be in check.
4) The square to which the king moves may not be attacked by the opponent.
5) The square between the king's current and new location may not be attacked by the opponent either.

In the diagram Black has lost his queen side castling rights because the rook an a8 has moved to c8. Even if he first plays Rc8-a8 he cannot castle queen sides (condition 1). Black has not yet lost his king side castling rights, but he is currently unable to perform king side castling because the square f8 is attacked by the White bishop on b4 (condition 5).

Castling must always be performed by first moving the king to either g1 or c1 (g8 or c8 for Black), and then moving the rook. In Sigma Chess you only have to move the king; the rook is moved automatically for you.

The castling move may seem a rather strange move, but it was introduced to allow the king to get into safety while at the same time getting the rook into play. As such castling is very useful and highly recommended and occurs in almost every game of chess.

## En Passant

The "en passant" move is a special pawn move where an opponent pawn is captured:

Black to move

The white pawn on c4 effectively blocks the advance of the black pawn on d6. If Black plays 1. ... d6-d5, White can capture the black pawn by playing 2. c4xd5. There is nothing strange about this.

The white pawn on g5 similarly blocks the advance of the black pawn on f7, except that here Black can play 1. ... f7-f5. This is where the en passant rule can be applied, allowing White to "capture" the pawn on f5 by playing 2. g5xf6EP. The black pawn is said to have been captured "en passant" (select "Redo Move" from the "Game" menu):
1 . . . f5
2 gxf6EP
En passant is only legal immediately after an opponent double move. If White had for instance played 2. Kd3-e3 instead he would lose the "right" to capture the black pawn on f5 en passant.

The en passant rule may seem like a strange move, but it was introduced in order to take away some of the advantages of double pawn moves, the reason being that it is somewhat unfair that the white pawn on g5 cannot stop the black pawn on f7.

## Draw by Repetition

As we have seen previously, a game of chess is drawn if there isn't sufficient material to force mate (e.g. king and bishop versus king, or simply king versus king). There are other situations though where a game is drawn even if one side is materially ahead as in shown in the diagram:

Black to move

Things look very bad for Black, because his bishop on f8 is subject to a devastating attack by the White queen and rooks. Black however saves the game as follows:
1 . . . Qe1+!
2 Kh2 . . .
Apparently Black hasn't achieved anything. His bishop is still subject to capture and he obviously cannot checkmate White.
2 . . . Qh4+
Again, White only has one move:
3 Kg1 . . .
It may now be tempting to capture the knight on c4. But this obviously loses after 4. Rc8xf8#. Instead Black plays:
3 . . . Qe1+!
And now we are back at the position after Black's first move, and there is nothing White can do about this.
4 Kh2 Qh4+
5 Kg1 Qe1+
Black can continue like this as long as he likes, giving checks on e1 and h4. So despite his great material advantage, White in fact never gets the opportunity to capture on f8. The game is therefore a draw.

By definition a game is a "draw by repetition" if the current position has occurred three times with the same player to move (not necessarily three times in a row):
6 Kh2
Draw by repetition, since this position has occurred two times previously after White's 2nd and 4th move. A draw by repetition can also occur in materially even positions, where neither side can make any progress.

## The 50 Move Rule

The 50 move rule states that a game is drawn if no pieces have been captured and no pawns have been moved in the last 50 moves.

White to move

In the diagram, White cannot win unless Black makes a mistake, despite White's material advantage. If it wasn't for the 50 move rule, White could play on almost endlessly (only bounded by the "Draw by repetition" rule which is, however, not very useful in this situation) hoping for Black to make a mistake.

## Draw Offers

A game is also drawn if both players agree on this. More precisely, a player can offer a draw immediately after performing his move. If the opponent accepts the draw offer, the game is considered drawn. The opponent must either accept or decline the draw offer before performing his move.

Note that a draw offer is actually considered part of the move after which the offer is issued. In a real over the board game with a human opponent, the draw offered should therefore be issued immediately after performing a move but before pressing the chess clock (Sigma Chess doesn't care about this; you can offer a draw at any time after your move when the program is thinking).

# 4. A Sample Game

We conclude this tutorial with a sample game where the various chess rules will be illustrated. The purpose here is not to play a strategically correct game.
1 d4 . . .
It is generally a good idea to gain control over the centre in the start of a game by playing d2-d4 or e2-e4. This also has the advantage of opening up for the bishops; in this case Bc1.
1 . . . d5
2 Nf3 . . .
White moves his knight out to enable castling as soon as possible.
2 . . . Nc6
3 e3 . . .
Opens up for the bishop on f1.
3 . . . Nf6
4 Bb5 . . .
Pins the knight on c6, which is now unable to move. By doing so the black king would be in check, and according to the rules it is illegal for a player to expose his king to check.
4 . . . g6
5 O-O . . .
White castles and brings his king into safety.
5 . . . Bd7
Unpins the knight on c6, since it can now move without exposing the black king to check.
6 c4 dxc4
7 Bxc4 Bg7
8 d5 . . .
Threatens to capture Nc6, which should therefore move eventhough Black could recapture. This is because the knight is stronger than a pawn.
8 . . . Nb8
9 Nc3 c5
Now the pawn on d5 can capture en passant:
10 dxc6EP Bxc6
11 Qxd8+ . . .
Black is now in check and only has one legal move:
11 . . . Kxd8
All other moves are illegal since the king is left in check.
12 Rd1+ . . .
Again Black is in check but here he has other options than moving the king. He can for instance also play 12. ... Nb8-d7 or 12. ... Bc6-d7.
12 . . . Ke8
13 Nd4 . . .
Note that Black has lost his castling rights because his king has already moved. In particular, Black cannot castle with the rook on h8 even though the squares f8 and g8 are both unoccupied.
13 . . . e6
14 Nxc6 Nxc6
15 Bb5 Ke7
16 Bxc6 bxc6
17 e4 Rhd8
18 Bg5 Rxd1+
White is check and must capture the black rook on d1.
19 Rxd1 e5
20 Bxf6+ Bxf6
21 f3 Rd8
22 Rxd8 Kxd8
In the resulting endgame the goal for each player is now to promote a pawn.
23 Kf2 Be7
24 Ke3 f5
25 exf5 gxf5
26 g4 fxg4
27 fxg4 Kd7
28 Ke4 Bb4
29 a3 Bxc3
30 bxc3 Ke6
Black must defend e5.
31 h4 a5
32 c4 h6
33 g5 hxg5
34 hxg5 c5
35 g6 Kf6
36 g7 . . .
White sacrifices this pawn. Black has to capture and then White can take e5 and c5.
36 . . . Kxg7
37 Kxe5 Kf7
38 Kd5 Ke7
39 Kxc5 . . .
Black is lost now.
39 . . . Kd7
40 Kb6 Kd6
41 c5+ . . .
Now this pawn will soon promote.
41 . . . Kd7
42 Kb7 Kd8
43 c6 a4
44 c7+ Ke7
45 c8=Q Kf6
46 Qc6+ Ke7
47 Qxa4 Kf6
48 Qe4 Kf7
49 a4 Kf6
50 a5 Kf7
51 a6 Kf6
52 a7 Kf7
53 a8=Q . . .
It is possible to have more than one queen.
53 . . . Kf6
54 Qf8+ Kg5
55 Qef4+ Kg6
56 Q4f5#
Checkmate.

This sample game concludes this tutorial of the rules of chess.