Playing a game

Surround plays Go on a 9×9, 13×13 or 19×19 board. Two players take turns placing stones on the intersections, black first. Stones never move once placed; they leave the board only when they're captured. The game is won by controlling more of the board (Scoring).

Who plays black

Seat 0 plays black and moves first; seat 1 plays white and receives komi. In a rated game the matchmaker's ticket names who sits where: at an open table the host plays black. Games are even: Surround has no handicap stones.

A turn

On your turn you do one of three things:

• Play a stone on an empty intersection, if the move is legal (below).
• Pass. Passing is always legal. Two passes in a row end play and start scoring.
• Resign, which ends the game at once. You can resign at any time, even on your opponent's turn.

Liberties and capture

The empty intersections next to a stone (up, down, left and right, not diagonally) are its liberties. Stones of one color that touch along the lines form a group and share their liberties.

A group whose last liberty is filled by the opponent is captured: all its stones come off the board.

Captured stones are counted and shown on the game record, but under Surround's area scoring they add nothing to the score by themselves: what counts is the board at the end.

Suicide

You may not play a stone that leaves your own group with no liberties.

Captures come first. If your stone takes the last liberty of an opposing group, that group is removed before your own liberties are checked. A move that captures is therefore never suicide.

Ko and positional superko

In the last diagram, Black would like to play straight back into the captured point, capturing the white stone that just took. That recreates the position from one move earlier, and the two players could trade captures forever. This shape is a ko.

Surround uses positional superko: no move may recreate any whole-board position that has occurred before in the game. Black has to play somewhere else first (a "ko threat"), and can retake on a later turn once the position has changed.

Superko covers every earlier position, not only the one just before, which also rules out longer cycles. Passes are exempt: a pass doesn't change the board, so it's always legal. The full position history is kept for the whole game, including across a scoring resumption.

Time

Casual games are untimed. Ranked games are timed by a referee clock, either 60 seconds a move or Japanese byo-yomi; running out loses the game. See Ranked clocks.

How moves are checked

Every move is signed by the player who makes it, and both players' clients check each move against these rules before accepting it. When a game reaches the chain, Cairo replays every move under the same rules, superko included. See Offchain play.