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Quantum Chess · Official rules

The rules of
Quantum Chess.

Every piece begins in superposition.
Every move is a measurement.

The quickest way to learnLearn on the board.Play the guided tutorial. Make the moves, watch the collapses, and learn each rule as it happens.Play the tutorial

Turn sequence

  1. Move and measureChoose a move allowed by a remaining identity. Keep the identities that can make that move.
  2. Resolve a captureIf an enemy is taken, resolve it as its cheapest remaining identity. It leaves the board but still counts in the chess set.
  3. Apply the censusEvery possibility must still fit one complete chess set per side. Remove identities that no longer fit.
  4. Resolve interactionFrom the landing square, use the mover’s lowest-value identity: Zap attacked enemies, then Heal protected allies. Recheck the census as these effects resolve.
  5. Check the positionIf the opponent has a piece with only King remaining and you attack its square, they are in check and must resolve it on their turn. If they have no legal reply, it is checkmate and you win. Stalemate and the other draw rules end the game in a draw. Otherwise, the other side moves.
The rulebook

Keep the rules at hand while you play.

Pieces & captures

Superposition & movement

Each piece begins with all six identities. Move as any identity still present; retain only those compatible with the move. White moves first, then turns alternate.

If you land on an enemy, capture resolves it as its lowest-value identity. Captured pieces still count toward the fixed chess set, explained next in the census.

Superposition

Movement is measurement

Every piece begins in a superposition of Pawn, Knight, Bishop, Rook, Queen and King.

Move as any identity still present. The move projects the piece onto the identities compatible with that displacement.

A knight leap resolves a piece as Knight. A clear diagonal move of two or more squares preserves Bishop and Queen.

On the board · Measure a diagonal move

Notice the six identities at d2. The highlighted diagonal is two squares long: only Bishop and Queen can make it.

At f4, only Bishop and Queen remain. The piece moved without choosing between those two identities.

White moves first; turns alternate. Paths, captures and pawn direction follow the movement rules of each remaining identity. Pawns may occupy the back rank. A possible Pawn can double-step from either of its two home ranks when the path is clear; this does not depend on whether the piece has moved before.

Capture

A captured state becomes definite

Capture resolves the victim as its lowest-value remaining identity.

A Knight–Queen superposition is captured as Knight. The resolved identity remains in the census after the piece leaves the board.

Resolution proceeds in order: movement → capture → census → interaction. The capturing piece then zaps enemies and heals friends from its landing square.

On the board · Capture a superposition

Watch d5: it is Knight–Queen. Move the Rook onto it to capture; compare this with a Zap, which leaves its target on the board.

The victim leaves d5 as a captured Knight, its cheaper identity. It still fills the Knight slot, so h7 becomes Queen.

The capture order is Pawn, Knight, Bishop, Rook, Queen, King. Capturing a superposition that includes King does not by itself win the game: the victim resolves as its cheapest identity. The ending rules below explain revealed Kings and checkmate.

Conservation

The quantum census

Each side must fit 8 Pawns, 2 Knights, 2 Bishops, 2 Rooks, 1 Queen and 1 King. Every living or captured piece occupies a slot in each admissible state.

The census runs after movement and capture, and again as interaction changes possibilities. Filling a slot in one place can rule out that identity elsewhere: a collapse cascade.

Census

The state belongs to the whole army

Every admissible state must contain one complete chess set per side.

The conserved slots are 8 Pawns, 2 Knights, 2 Bishops, 2 Rooks, 1 Queen and 1 King. Captured pieces still occupy their slots.

When one superposition changes, the census removes identities that no longer fit elsewhere. This is a collapse cascade: a distant piece can resolve without being directly attacked.

On the board · One move resolves a distant piece

Both c1 and h2 are Pawn–Knight. Watch h2 while the other piece makes a knight leap.

The leap fixes d3 as Knight. That fills the last Knight slot, so distant h2 becomes Pawn without being attacked or protected by the mover.

Two resolved Knights exclude Knight from every other unpromoted piece on that side. Constraints can also arise from groups: three Bishop–Queen pieces occupy both Bishop slots and the Queen slot. A closed group does not provide an automatic shield against a zap.

Interaction

Zap & Heal

Interaction connects two familiar chess ideas: attacking an enemy and protecting an ally. After a move, enemies in the mover’s interaction pattern are Zap targets; allies in that pattern are Heal targets. Use the capture pattern of only the mover’s lowest-value identity: Pawn → Knight → Bishop → Rook → Queen → King.

Zap models quantum measurement: it narrows an enemy’s possibilities. Heal models quantum state preparation: a friendly interaction restores a possibility. Resolve enemy Zaps first, then friendly Heals. Each group resolves jointly with the census. Both effects are deterministic.

Attack an enemy · ZapProtect an ally · HealNo removal · Shield
Zap

Zap models measurement

Zap is the effect of an attack through the mover’s interaction pattern. It attempts to rule out one enemy identity while leaving the piece on the board, often with several possibilities still open.

After movement, the piece interacts using the capture pattern of its lowest-value remaining identity. An enemy in that pattern is attacked; a friendly piece in it is protected. The order is Pawn → Knight → Bishop → Rook → Queen → King.

Enemy interaction attempts to remove the highest-value identity: King first, then Queen, Rook, Bishop, Knight and Pawn. The remaining joint state must satisfy the census.

On the board · Attack a piece without capturing it

Move the Knight to e4. From there it attacks f6, so f6 is zapped even though the Knight never lands on it.

The red target at f6 loses Queen and remains on the board as Rook. The census then fixes distant h8 as Queen.

All zaps resolve as one volley. When removals conflict, retain the largest admissible set, then prefer higher-value identities and earlier squares in algebraic order. If the volley would remove every remaining King possibility, affected targets retry below King. A zap can propagate census collapse to pieces outside the interaction geometry.

Heal

Heal models state preparation

Heal is the effect of protecting an ally through the mover’s interaction pattern. It attempts to add a missing identity back into that ally’s superposition, opening another possible way to move.

A heal restores a missing identity that the joint census permits, starting with the lowest value.

Several heals may support one another. They resolve jointly, restoring as many affected pieces as possible.

On the board · Protect allies to restore possibilities

The Knight on f4 will protect both h5 and d5. Watch their identities; neither ally needs to move.

Both allies become Pawn–Bishop: h5 regains Pawn, and d5 regains Bishop. Joint healing allows them to exchange census assignments.

The usual order is Pawn, Knight, Bishop, Rook, Queen, King; equal-size healing combinations prefer lower-value regains in board order. If a side has no King possibility, King is attempted first. Pawn cannot return to a promoted piece or a piece on its promotion rank. No admissible regain means the state remains unchanged.

Castling, en passant & promotion

Special moves

Castle two back-rank superpositions that each contain Rook and King. En passant projects both pieces onto Pawn. Promotion transforms the Pawn branch while preserving its Pawn census slot.

These moves obey the same conservation rules as the rest of the game. Open a rule below for its conditions.

Castling

A joint Rook–King projection

Once per game, select two back-rank superpositions that each contain both Rook and King.

Click one eligible piece, then the other. The interval between them must be clear and contain at least one empty square.

The pieces form a central adjacent pair and project onto Rook–King. The census then constrains the joint state.

On the board · Bring a Rook–King pair together

Both a1 and e1 include Rook and King, and their back-rank interval is empty. Select the pair to castle.

The pair moves to c1 and d1. Both remain Rook–King; the census allows exactly one to be King. This uses White’s one castle.

Previous movement does not remove eligibility. With only one empty square between the pieces, one starting square is retained. Castling never captures and consumes the turn. It must respect the census and cannot leave a revealed King capturable. Later interaction can heal the castled pieces normally.

En passant

A joint Pawn measurement

A possible Pawn that double-steps opens an en passant window for one turn.

An adjacent possible Pawn may capture diagonally into the crossed square. The double-stepping piece is removed from its landing square.

Both pieces project onto Pawn. The capturer then interacts from its new square.

On the board · Capture through the crossed square

First play Black’s d7 → d5. Watch the empty square d6 that the Pawn crosses.

White lands on d6, while the Black Pawn disappears from d5. Both pieces are resolved as Pawn, including the captured victim.

The double-step begins on either of the mover’s two home ranks, along a clear file. The capturer must be beside the arrival square, and the crossed square must be empty. If an ordinary move to that square is also legal, choose whether to perform the en passant measurement.

Promotion

The Pawn branch transforms

At the farthest rank, a Pawn branch becomes a superposition of Knight, Bishop, Rook and Queen.

Other identity branches remain. Each promotion-derived identity still occupies a Pawn census slot.

The bar under a promoted piece marks this branch. Promotion occurs once per piece; healing cannot restore Pawn afterward.

On the board · Transform the Pawn branch

Move the Pawn from c7 onto White’s farthest rank. Watch both the identities and the promotion marker.

At c8 the Pawn becomes Knight–Bishop–Rook–Queen. The bar under the piece marks its Pawn origin: four movement possibilities, still one Pawn census slot.

Promotion-derived branches remain constrained by the Pawn pool. If all eight Pawn slots are claimed elsewhere, those branches are excluded. On capture, the piece resolves as its lowest-value remaining identity while retaining its census origin.

Shields & ending the game

Shields, checkmate & draws

A revealed King is a piece whose only remaining identity is King. It is in check when an enemy can capture its square. A piece that still mixes King with other identities is not subject to check. Checkmate wins the game.

A gold shield means the direct Zap removes no identity. It can appear when attacked pieces start in superposition, and always appears when a target has only one identity left. It does not prevent capture, check or checkmate. Explore the examples below.

Shields in superposition

A shield means the direct Zap removes nothing

A gold shield can appear on a piece that was in superposition when attacked. It marks a direct Zap that cannot join the successful removals while preserving the census and at least one King possibility.

Zaps resolve together. If attacking every remaining possible King would remove King everywhere, those targets try their next-highest identities instead.

Two Queen–King pieces sharing one Queen slot and one King slot illustrate this. When both are attacked, both try to lose Queen instead of King. They cannot both become King: one Zap succeeds, and the other target shields.

On the board · Attack two superpositions and see a shield

Move the Knight to d4, where it attacks both Queen–King pieces. Watch f3 for a gold shield.

Black moves its King to a3, out of the Knight’s attack. Play continues. The shield at f3 stopped its direct Zap, but the census still resolved it as Queen. Check only becomes checkmate when there is no legal reply.

In this example, the successful Zap resolves one piece as King, so the census resolves the shielded piece as Queen. The pieces start in superposition but do not stay that way. A shield blocks a direct Zap; it does not prevent a census collapse or a capture. Being part of a constrained group does not automatically grant a shield.

Shields with one identity

Zap cannot erase a piece

A piece with just one identity left always shields against Zap: removing that identity would leave no piece.

This applies to Pawns, Knights, Bishops, Rooks, Queens and Kings. A definite Rook stays a Rook when zapped; a definite King stays a King.

The shield only stops the Zap effect. An enemy still attacks the square and can capture the piece with a legal move onto it. A shielded King can still be in check or checkmate.

On the board · One identity left: nothing to Zap away

Move the Knight to e4. It attacks the Rook at f6 without landing on it.

The Rook at f6 shows a gold shield and stays a Rook. Removing its last identity would erase the piece, which Zap cannot do. The Knight still attacks f6: the shield does not prevent a later capture.

Checkmate requires check and no legal reply. If a checked King has a legal escape, play continues. If the side to move has no legal move but its King is not in check, the result is stalemate. The following boards compare checkmate with stalemate.

King & checkmate

A shield does not stop checkmate

Zap cannot remove the final King possibility. Check applies only to a piece whose sole remaining identity is King: a revealed King.

For example, a Rook–King piece is still in superposition: attacking it is not check. Once only King remains, an enemy threatens check if any of its remaining identities can capture that square, even if the attacker is still in superposition.

Every move is checked after movement, capture, census, Zap and Heal resolve. If your King is revealed in the resulting position, no enemy may be able to capture its square. This also applies when your move reveals your King.

If you are in check, your next move must resolve it. Moving the King to safety, blocking the attack or capturing the attacker can do this, provided the final position passes the same safety check. Check with no legal reply is checkmate.

On the board · A shield does not stop checkmate

Move the Rook to a6. Notice the Black King at a8 and its three possible escape squares: a7, b7 and b8.

The King has only one identity left, so it shields against Zap. The Rook still checks it along the a-file and covers a7. White’s King covers b7 and b8. Black has no legal escape: checkmate, despite the shield.

When a proposed volley would erase every King possibility, those targets retry below King. A definite King has no lower identity to lose, so it shields. A shield blocks the Zap effect; it does not stop an attack, check or checkmate. If another resolution leaves a side temporarily without a King possibility, play continues; a later heal can restore King.

Draws

When the position stops evolving

Stalemate, threefold repetition and the fifty-move rule produce a draw.

Stalemate means no legal move when the side to move is not in check. Repetition compares the full quantum state, including the side to move and en passant window.

The fifty-move clock resets on a capture, a definite Pawn move, promotion, or a net reduction of possibilities across the board.

On the board · No move, but no check

Move the Queen to b6. Watch a8 and the escape squares a7, b7 and b8.

The Queen controls a7, b7 and b8, but does not attack a8, so the King receives no Zap or shield. Black has no legal move and is not in check: stalemate, a draw, not checkmate.

The limit is 100 half-moves without one of those changes. Repetition also includes promotion branches and castling state. A heal alone does not reset the clock. Players may agree a draw or resign.

Ready to make the first measurement?Play the tutorial