Chess in the dark

Kriegspiel is chess with the lights off. You sit at a board holding only your own sixteen pieces. Your opponent sits at another, holding only theirs. Between you, a referee keeps the one board that is real — and tells you almost nothing about it.

It was invented in 1899 by Michael Henry Temple, who named it after the Prussian army's war game. The joke of the name is the joke of the game: generals don't get to see the enemy either. They send scouts, they lose them, and they learn where the enemy is by finding out where they aren't.

Nothing about how the pieces move changes. A knight still moves like a knight. Checkmate still ends it. The only thing that changes is that you can't see half the board — and that turns out to change everything.

What you actually see

Your pieces, and nothing else. The other thirty-two squares are fog. Somewhere in that fog is a queen that would very much like to meet your king.

So you don't play by looking. You play by listening. The referee has a small, fixed vocabulary — six things, really — and every word is a gift. So is every silence.

Everything the referee will ever say

Both players hear all of it. Neither player is ever told which piece did anything.

“Black to move.”

A move happened. It captured nothing, it checked nobody. That is all you get, and it is most moves.

“Pawn captured on e4.”

Something took a pawn on e4. You are told the square, and whether the victim was a pawn or a piece — never which piece, and never what took it.

“Piece captured on f7.”

Same, for anything that isn't a pawn. If it was your piece, you know exactly what you lost. If it was theirs, you now know something they wish you didn't.

“Check on the file.”

Your king is attacked from along its own file. Also: on the rank, on the long diagonal, on the short diagonal, by a knight. You get the line, never the square.

“Check by a knight and on the rank.”

A double check. Two announcements at once, which is the referee's way of telling you that only your king can save you now.

“No.”

The move you just tried is illegal on the real board. Take it back and try another. It costs you nothing — but your opponent hears that you bumped into something.

What would the referee say?

The full position is shown here — you'd never see it. Guess, then reveal. The captions are generated by the same referee code that runs the game, so they don't lie.

“Any?” — your radar

On your turn, before you move, you may ask the referee one question: “Are there any pawn captures?” Everyone says just “Any?”

• If no pawn of yours can capture anything, the referee says “No.”
• If at least one can, the referee says “Try!” — and now you must attempt a pawn capture as your move.
• If your attempt turns out to be illegal, fine: you've paid the price, and you may now play anything.
• Ask with no pawns on the board and the referee will say “Hell no.” The referee has been doing this a long time.

This is the single densest source of information in the game. A “Try!” means an enemy man is standing on one of a handful of squares — the ones your pawns attack. A “No” is just as loud: it means every square your pawns attack is empty. Push a pawn to the middle, ask, and you have swept a two-square arc of the fog.

Illegal moves are free. They are not secret.

Try to slide your rook across a square where an invisible knight is standing, and the referee says “No.” You lose no time; you simply move again. You have also just learned that something of theirs is sitting on that square, or on one before it.

This is called probing, and it is a real tactic. It is also a real leak: the referee announces that you tried an illegal move, so your opponent knows you found something. Good players probe when the information is worth more than the noise.

What the referee never tells you

Three of chess's biggest events happen in total silence:

• Castling. The king and rook swap corners and nobody says a word.
• Promotion. A pawn reaches the eighth rank and becomes a queen. Silence. That new queen may spend ten moves before anyone learns it exists.
• En passant. Announced as an ordinary capture, on the square the capturing pawn lands on — not the square the victim was standing on. Read that twice; it has confused stronger players than us.

Count the enemy's pieces from the announcements and you may still be wrong. That is the game.

How to actually play

1. Assume nothing is where you left it. The most common way to lose a queen is to remember the board as it was four moves ago.
2. Silence is information. A quiet move means whatever moved didn't take anything and didn't check you. It went somewhere harmless — so the squares near your king are, for one moment, safer.
3. Advance pawns in a wall. Pawns are your radar dish. The more squares they cover, the more “Any?” is worth.
4. Keep your king boring. Tuck it in a corner behind its own pawns. A king in the centre of a fogged board is a king you will lose to a piece you never saw.
5. When you hear a check, don't panic — triangulate. “Check on the short diagonal” plus your knowledge of your own pieces usually leaves two or three candidate squares. Block one. If the referee says “No,” you have found it.
6. Trade when you're ahead, hide when you're behind. Every capture is announced, and every announcement burns away fog. Fog is the resource of the losing side.

How the panda plays blind

The engine does not look at the board. It genuinely can't — the true position lives in a closure that only the referee holds, and the engine is handed nothing but the same announcements you get.

It keeps a crowd of guesses

Instead of one position, the engine holds a pool of dozens of complete imagined positions — "particles". In every one of them, its own pieces sit exactly where they really are. Yours are invented.

Then it listens, and the crowd thins:

• You move quietly → every particle must produce some legal move of yours that would have made exactly that silence. Those that can't are wrong about the world, and die.
• You capture on e4 → only particles where one of your men could reach e4 survive.
• The engine tries a move and hears “No” → every particle in which that move was legal is now known to be false. Deleted.
• The engine asks “Any?” and hears “No” → every particle where it would have had a pawn capture is a lie. Deleted.

This is a particle filter, the same idea a robot uses to work out where it is in a building from a bad map and a worse sensor. The literature calls the Kriegspiel version "game-tree search with combinatorially large belief states" (Parker, Nau & Subrahmanian) and the strongest published engines — Ciancarini & Favini's Darkboard, and the Penumbra program that won Reconnaissance Blind Chess — are elaborations of it.

The pool bleeds, so it gets refilled

Filtering only ever removes hypotheses. Left alone, the pool shrinks to one lucky guess and then to zero, when some announcement is impossible in every world it believes in. So each turn, the engine invents fresh armies from scratch — placing your pieces at random, then throwing away any arrangement that contradicts what it has legally heard: the tally of captures it has been told about, the check it can hear right now, whose move it is.

That reconstruction uses only public knowledge, which is why it isn't cheating. Turn on “Show what the engine believes” during a game and you can watch the cloud: violet squares are where the crowd thinks your pieces are. Early on it's nearly right, because the opening is common knowledge. Later it is often gloriously wrong.

Choosing a move it cannot be sure is legal

A move is scored by playing it in each imagined world and averaging. Some worlds reject it outright — a pawn is in the way that may or may not exist. So a move's value is weighted by how often it is even possible: a brilliancy that works in a third of the worlds and is illegal in the rest is a third of a brilliancy.

Illegal attempts cost no time in Kriegspiel, only secrecy. A purely greedy engine therefore probes relentlessly and hands you a map of its findings. Ours is charged a fee in centipawns for every bump, and the stronger levels mind the fee more.

The levels

Strength is not search depth here — it's how many worlds it can hold in its head at once.

The ratings are rough labels, not measured Elo. Kriegspiel strength doesn't map cleanly onto chess strength: a 1600 chess player meeting Kriegspiel for the first time will lose to Cadet, and know exactly why, and lose again.