What is Phantom Go?
Phantom Go is Go with hidden stones. Each player sees only their own stones; the real board, with both colours on it, is kept by a referee. Here the table is the referee. You choose a point as usual, and if the move is legal on the real board it is played. If it is not, because the point is already taken by a stone you cannot see, or the move would be suicide or break the ko rule, the referee says only that the move is illegal, without saying why. You stay on turn and try another point.
Every refused point is marked on your board, so you build up a picture of where the opponent's stones must be. Each point can be tried only once per turn, so every turn ends. Your opponent hears that a try was refused, but not which point. When stones are captured, the points they stood on are announced to both players, but the point of the stone that captured them is not. Passes are announced too.
This table follows the referee rules used for Phantom Go at the Computer Olympiad: the referee answers only legal or illegal, captures are announced, and after two passes in a row the whole board is revealed and scored by area (Chinese rules). The usual board is 9×9, but every size the table offers works, and Phantom Go combines with handicap stones, Capture Go and the toroidal board.
You can play it online, against the computer or with a friend on one device. On one device a curtain covers the board between turns, so each player sees only their own stones.
Rule changes
| Rule | Standard Go | Phantom Go |
|---|---|---|
| What you see | Standard GoBoth players and any spectators see every stone on the board. | Phantom GoYou see only your own stones, plus any handicap stones, which everyone knows. Spectators see only the handicap stones until the game ends. |
| Illegal moves | Standard GoYou can see which points are taken, and an illegal move simply can't be played. | Phantom GoA move onto an occupied point, a suicide or a ko violation is refused with no reason given. You stay on turn and try another point; each point once per turn. The opponent hears only that a try was refused, not where. |
| Captures | Standard GoStones without liberties are removed, and everyone sees which. | Phantom GoStones without liberties are removed, and the points they stood on are announced to both players. The point of the capturing stone stays hidden. |
| Passing and the end | Standard GoTwo passes in a row end the game. | Phantom GoPasses are announced to both players, and two passes in a row end the game. Then the whole board is revealed. |
| Komi | Standard Go6.5 points with Japanese scoring or 7.5 with Chinese scoring on 19×19, and 5.5 on smaller boards. | Phantom Go7.5 points on every board size, or 0.5 with handicap stones. You can choose a different komi in the table setup. |
| Scoring | Standard GoJapanese (territory) or Chinese (area) scoring, chosen in the table setup. | Phantom GoAlways area scoring (Chinese rules) on the revealed board: your stones plus the empty points you surround, plus komi for White. |
History
Phantom Go applies to Go the idea of Kriegspiel, the chess variant in which each player sees only their own pieces and a referee keeps the real board and answers whether a move is legal. Played face to face, it needs three boards: one for each player behind a screen and one for the referee.
The game drew the attention of computer Go researchers in the 2000s, because a program has to play well without knowing the position. Tristan Cazenave described a Phantom Go program built on Monte Carlo methods in his paper A Phantom-Go Program, presented at the Advances in Computer Games conference in 2005. Phantom Go then became an event of the Computer Olympiad: Cazenave and Joris Borsboom reported on the 2007 tournament in Amsterdam, won by Cazenave's program Golois, and the event was held again at later Olympiads.