Chess Rules

Complete reference for the FIDE 2023 Laws of Chess as implemented by CheckAI.

Piece Movement (FIDE Art. 3)

General Rules

• No piece may move to a square occupied by a friendly piece
• Moving to a square with an opponent piece captures it
• Bishops, rooks, and queens cannot jump over other pieces
• Knights are the only pieces that can jump
• No move may leave or place your own king in check

King (K / k)

• Moves exactly one square in any direction
• May not move to an attacked square
• Special: castling (see below)

Queen (Q / q)

• Moves any number of squares along a file, rank, or diagonal
• Combines rook and bishop movement
• Cannot jump

Rook (R / r)

• Moves any number of squares horizontally or vertically
• Cannot jump

Bishop (B / b)

• Moves any number of squares diagonally
• Always stays on its starting square color
• Cannot jump

Knight (N / n)

• Moves in an L-shape: two squares in one direction, then one square perpendicular
• Jumps over other pieces
• From e4: possible targets are d6, f6, g5, g3, f2, d2, c3, c5

Pawn (P / p)

Forward movement (non-capturing):

• One square forward to an empty square
• Initial double-step: From the starting rank (2 for White, 7 for Black), may move two squares if both are empty

Capturing:

• Diagonally one square forward
• Cannot capture straight forward

En passant:

• If an opponent pawn double-steps past your pawn, you may capture it as if it moved one square
• Must be done on the very next half-move or the right expires
• The target square is given in the en_passant field

Promotion:

• A pawn reaching the last rank must be promoted to Q, R, B, or N
• Multiple queens (or other pieces) are allowed

Castling (FIDE Art. 3.8.2)

Castling is a combined king-and-rook move that counts as one king move.

Procedure

Type	King	Rook
White O-O	e1 → g1	h1 → f1
White O-O-O	e1 → c1	a1 → d1
Black O-O	e8 → g8	h8 → f8
Black O-O-O	e8 → c8	a8 → d8

Permanently Impossible If

1. The king has already moved
2. The corresponding rook has already moved

Temporarily Prevented If

1. Squares between king and rook are occupied
2. The king is currently in check
3. The king passes through an attacked square
4. The king's destination is attacked

Mnemonic: The king may not castle out of, through, or into check. The rook may pass through or land on attacked squares.

Check, Checkmate, Stalemate

Check

• The king is attacked by one or more opponent pieces
• Must escape check immediately by:
  • Capturing the attacker
  • Blocking the attack
  • Moving the king

Checkmate — Win

• The king is in check with no legal escape
• The checkmated side loses

Stalemate — Draw

• The side to move is NOT in check but has no legal move
• The game ends as a draw

Draw Conditions (FIDE Art. 5 & 9)

Threefold Repetition

• Same position occurring 3 times → claimable draw
• Same position occurring 5 times → automatic draw
• "Same position" = identical pieces, same side to move, same castling rights, same en passant

50-Move Rule

• 50 moves by each side (100 half-moves) with no pawn move or capture → claimable draw
• 75 moves (150 half-moves) → automatic draw (unless the 75th move is checkmate)
• Check via halfmove_clock: ≥ 100 = claimable, ≥ 150 = enforced

Insufficient Material (Dead Position)

Automatic draw when checkmate is impossible:

Position	Result
King vs King	Draw
King + Bishop vs King	Draw
King + Knight vs King	Draw
King + Bishop vs King + Bishop (same color)	Draw

INFO

Two knights vs king is not considered a dead position — checkmate is possible with opponent cooperation.

Draw by Agreement

Both sides may offer and accept a draw via special actions.