Cat Chess

Full rules

1. Board and pieces

A standard chessboard and the standard chess position for White and Black. The Cats have two pieces, which start on a4 and h5.

2. Turns

White moves first, then Black, then the Cats, always in that order. Every turn someone must move: on the Cats' turn, that can be a Cat staying where it is.

3. White and Black

Their pieces move and capture exactly as in chess, including castling, en passant and promotion to queen, rook, bishop or knight, except where the Cats forbid a capture (sections 5 to 7).

4. The Cats

• On the Cats' turn, one Cat moves one square in any direction, like a king, or the Cats pass.
• A Cat may only move onto an empty square, but it may move onto a square that is under attack.
• A Cat never captures, and nothing can capture a Cat.
• A Cat occupies its square, so it blocks the lines of rooks, bishops and queens.

5. The no-capture zone

Each Cat keeps a zone: its own square and the eight squares around it. Zones move with their Cats and are worked out afresh after every move.

6. Disarmed pieces

A White or Black piece standing in a zone is disarmed. A disarmed piece:

• can still make any ordinary, non-capturing move, including out of the zone, which re-arms it;
• cannot capture, and cannot be captured;
• attacks no squares, so it cannot give check or checkmate;
• still blocks lines of attack, since it is still on its square.

7. The capture rule

A capture is illegal if the capturing piece starts in a zone, or if the piece being captured stands in one.

8. Check and checkmate

• A king is in check when an armed enemy piece attacks its square.
• A king inside a zone is never in check, since it cannot be captured there.
• A king may not move onto a square an armed enemy piece attacks, unless that square is inside a zone. No move may leave your own king in check.
• A Cat's move can disarm a piece that was giving check, or, by walking away, re-arm one and put a king in check. Kings are never captured: a king left open is simply in check when its own side's turn comes.
• Checkmate: the side to move is in check and has no legal move. Its opponent wins.

9. Draws: the Cats win

Any draw is a win for the Cats. The game is drawn when, at the start of a turn:

• White or Black is to move, is not in check, and has no legal move (stalemate);
• a hundred turns in a row have passed without a capture or a pawn move, counting the Cats' turns too;
• the same position, with the same side to move, has come up for the third time;
• only the two kings are left, or a king and a single bishop or knight against a lone king.

10. Who wins

• White checkmates Black: White wins.
• Black checkmates White: Black wins.
• Any draw: the Cats win.

Credits and licensing

• Cat Chess by Peace Cat Games: rules, Python engine and computer players in peace-cat-games/cat-chess.
• Browser version by Peace Cat Games, ported from that engine to TypeScript and checked against it.
• License: Apache License 2.0
• Source: peace-cat-games/cat-chess on GitHub

Notes on these rules

These rules follow the original Python engine, including the owner's latest change (a king may step onto an attacked square inside a zone), and its RULES.md. Three things here are deliberately different:

• Site rule: kings are never captured. In the original, when a Cat walks away from a king, the enemy can capture that king on the next turn, and play then carries on without it. Here the king is only in check, and must get out of it on its own turn. This is a house rule until the designer decides.
• Correction: repetition. The original compares a position recorded before the turn passes with the position after it, so a threefold repetition is rarely noticed. Here a position counts as repeated when the same pieces stand on the same squares with the same side to move; the starting position counts.
• Correction: castling rights. Here, as in chess, the right to castle on a side is lost when the king moves, or when that rook leaves its corner or is captured there. The original drops it whenever any rook leaves the a- or h-file, and keeps it when the rook is captured.

Kept as the original has them, but open questions for the designer:

• En passant. The chance lasts for one turn only, and the Cats' turn falls between Black and White, so Black can capture en passant but White never can.
• The “fifty-move” rule counts a hundred turns including the Cats', which is about 33 moves each by White and Black, not 50.
• Castling near a Cat. The king may not cross or land on an attacked square even inside a zone, although an ordinary king step onto such a square is allowed.
• Dead positions. Only bare kings, or a king and one bishop or knight against a lone king, end the game for lack of material. RULES.md also lists “no possible progress” as a draw; neither engine detects it.

About the original web page

The repository's single-page web version had fallen behind its Python engine: its Cats could not pass, a king inside a zone could still be in check, a king could not step onto an attacked zone square, and its computer players were simpler (material only, and a Cats player without the balance and low-mobility terms). This site follows the Python engine instead.

How this version was made

• The rules and computer players were ported from the original Python to TypeScript, keeping its move order so the computer breaks ties the same way. The port passes the original's own tests, and agrees with the Python engine, adjusted only for the three changes above, on the legal moves, checks and results of 1,669 positions from random games, and on 41 computer decisions.
• Chess symbols are set in Noto Sans Symbols 2, under the SIL Open Font License. The Cat drawing is our own.
• The original code is licensed under the Apache License 2.0; the ported files say so, and carry a notice that they have been changed.