The rules of chess

In brief

Two players, 16 pieces each, on a board of 8 × 8 squares. White moves first.
Each piece has its own way of moving. A move that leaves your own king in check is illegal.
You win by checkmating: the opponent's king is in check and no move gets it out of check.
Three special moves: castling, en passant capture and pawn promotion.
The game is drawn by stalemate, by insufficient material, by repetition, or after a long series of moves with no capture and no pawn move.

These rules are those of the app, which follows the Laws of Chess of FIDE, the international chess federation (see the sources). Each diagram was checked twice: by the app's rules engine and by a second, separately written program.

starting square of the move
destination square
king in check

The board and the starting position

The board has 64 squares, alternating light and dark, with a light square in each player's bottom-right corner. The files are named a to h, and the ranks are numbered starting from White's side: every square has a name, such as "e4".

Each side has 16 pieces: one king, one queen, two rooks, two bishops, two knights and 8 pawns. The queen goes on the square of her own color, next to the king.

On the first move, White has 20 possible moves: each pawn advances one or two squares, and each knight has two jumps.

After one move by each side, there are already 400 different move sequences; after White's next move, 8,902. These numbers are recalculated by the app's rules engine.

How the pieces move

Piece	Letter in the app	Movement
King	K	One square in any direction. It can never move to a square that is under attack.
Queen	Q	Any number of squares in a straight line or diagonally, without jumping.
Rook	R	Any number of squares in a straight line, without jumping.
Bishop	B	Any number of squares diagonally, without jumping: it always stays on the same color.
Knight	N	In an L shape: two squares in one direction, then one to the side. It is the only piece that jumps over others.
Pawn	none	Moves straight ahead one square, or two from its starting square; captures diagonally, forward. It never moves backward.

You capture an opposing piece by moving onto its square. The letters are those of English notation, used by the app and by the PGN format: K for king, Q for queen, R for rook, B for bishop, N for knight. A move is written with the piece's letter and the destination square, such as "Nf3"; a pawn move with the square alone; an x marks a capture, a + a check and a # checkmate.

Check and checkmate

A king that is attacked is in check. The player must then get out of check: move the king, capture the attacking piece, or place a piece in between. If none of these is possible, the player is checkmated and loses the game. A player may also resign.

White plays Qxf7#: the queen, protected by the bishop, captures the pawn and checks the black king. No move saves it: it's checkmate. Of the 43 legal moves, it is the only one that gives checkmate.

Castling

Castling moves both the king and a rook in a single move: the king goes two squares toward the rook, then the rook jumps to the other side of the king. Kingside castling is written "O-O", queenside castling "O-O-O".

It is allowed only if all these conditions are met:

the king and that rook have never moved since the start of the game;
the squares between them are empty;
the king is not in check, does not pass through any attacked square and does not land on an attacked square.

Nothing hinders the white king: both castling moves are allowed, O-O and O-O-O.

The black rook attacks the square the king would have to cross for kingside castling: it is not allowed. Only queenside castling, O-O-O, remains.

En passant capture

When a pawn advances two squares and stops right next to an opposing pawn, that pawn can capture it as if it had advanced only one square. This capture is allowed only on the move immediately following; after that, the chance is lost.

The black pawn has just advanced two squares, next to the white pawn. White can capture it en passant: exf6. The black pawn on the other side, which arrived earlier, can no longer be captured this way.

Promotion

A pawn that reaches the last rank is immediately replaced, of your choice, by a queen, a rook, a bishop or a knight of its color: 4 choices, offered by the app. A queen is most often chosen, but not always.

Here, the allowed promotions are f8=N+, f8=B, f8=R and f8=Q. Promoting to a queen (f8=Q) would leave Black with no legal move: stalemate, so the game is drawn.

Stalemate and draws

Stalemate

If the player to move has no legal move and their king is not in check, it is stalemate: the game is a draw.

Black is to move. Their king is not in check, but every square it could go to is attacked and they have no other piece: no legal move, so it is stalemate.

Insufficient material

The app declares a draw immediately when there are not enough pieces left to deliver checkmate:

king versus king;
king and a single minor piece (bishop or knight) versus king;
only bishops besides the kings, all on squares of the same color.

King and knight versus king: automatic draw.

One bishop for each side, both on dark squares: automatic draw.

King and two knights versus king: no automatic draw, the game continues. Checkmate is very rare, but possible if the lone side blunders.

Repetition and the fifty-move rule

Situation	Draw on claim	Automatic draw
The same position occurs again	3 times	5 times
Moves by each side with no capture or pawn move	50 moves	75 moves

“On claim”: the “Claim draw” button appears in the game menu, and the player decides. For example, if both knights go out and come back, the starting position occurs a 3^rd time after 8 half-moves: a draw can be claimed. If the players keep going, it happens automatically on the 5^th occurrence, after 16 half-moves. Likewise, the counter of moves with no capture or pawn move allows a claim as soon as it reaches 50 moves, and the draw is automatic as soon as it reaches 75 moves.

Players can also agree to a draw. With two players on the same device, the menu offers “Draw by agreement”; in a game on the same Wi-Fi, “Offer draw”. Against the computer, there is no draw by agreement.