Go

Black plays first. Stones go on the CROSSINGS rather than in the squares, including the ones on the outside lines, and once a stone is down it does not move again.

Every stone needs a liberty: an empty crossing next to it, up and down or left and right, never diagonally. Stones of one colour that touch each other form a group and share their liberties. Fill the last liberty of an enemy group and the whole group comes off the board.

You may not play a stone that would leave its own group with no liberties — unless the same move captures something first, in which case it is perfectly legal, and it is one of the commonest moves in the game. You also may not recreate a position that has already occurred, which is what stops two players taking the same stone back and forth for ever.

When neither player wants to add anything, both pass. That ends the play, not the game: you then agree which stones are dead, and the larger area wins.

Zerdly’s preset, stated rather than left to be discovered. Suicide is illegal — Chinese, Japanese and AGA rules agree, and only New Zealand and Tromp-Taylor allow it. Ko is positional superko: no move may recreate ANY board position that has occurred before, not merely the previous one. Simple ko is the rule most people are taught; superko is the one that actually ends every dispute. Scoring is by area, the Chinese count: your stones on the board plus the empty points they surround. A point touching both colours belongs to neither, which is what makes seki come out right with no special case.

Komi compensates white for moving second and always carries a half point, so a game cannot be drawn. The defaults are 5.5 on 9×9, 6.5 on 13×13 and 7.5 on 19×19, and you can change it. A handicap places black stones on the star points before white’s first move; you play black, which is what makes a handicap a handicap.

Show coordinates labels the lines in standard Go notation, which skips the letter I — a 19-wide board runs A to T. Show score estimate shades the board as it stands with nothing marked dead; it is off to begin with, because that is right at the end of a game and can be badly wrong in the middle, and it is labelled an estimate wherever it appears. Undo takes back your move and the computer’s reply. Sound can be turned off at any time; nothing here is conveyed by sound alone. A game in progress is saved as you play and is waiting when you come back — including one you are losing.

Versus the computer gives you five strengths, and they differ in one honest way: how many complete games the opponent plays out in its head before each move, from a few hundred to forty thousand. There is no weakening trick and no deliberate mistake — a weaker level simply thinks about fewer possible games.

Two players, one device takes the computer out of it, and it is where the full 19×19 board belongs. These games are counted but not scored: both players are here, so there is nobody for Zerdly to call the winner.

Capture Go changes the objective rather than the rules: first capture wins, and nothing else counts. Same board, same liberties, same ko. It is the version Go is taught with, and a game takes about two minutes.

Play in the corners first, then the sides, then the middle. It takes far fewer stones to surround ground in a corner, because two of the walls are already there.

Count liberties before you fight. A group with two liberties is in trouble and a group with one is already lost unless it can run, connect or capture — and the difference between two liberties and three decides most of the fights on the board.

Two eyes cannot be killed. A group surrounding two separate empty points can never have both filled, because filling either one would be suicide, so it lives no matter what happens around it. Almost every life-and-death problem in Go is really the question of whether a group can make two.

The whole game runs in your browser. The opponent thinks on your own device, in a background thread so the page stays responsive while it works — which matters here more than anywhere else on this site, because a single move can be several seconds of playing complete games out to the end.

Your record, your saved game and your sound, motion and assist settings are stored in this browser only. Nothing here asks for a name or an email, and the button on this page removes all of it — and only this game’s share of it.

Is it really free, and do I need an account?

Yes, and no. There is no sign-up, no trial and nothing to buy. The game runs in your browser and never sends your moves anywhere.

I have never played Go. Where do I start?

Capture Go, on the 9×9 board. First capture wins and nothing else counts, so a game takes two minutes and you can see exactly why you lost. It is how Go is taught in clubs and schools everywhere, and it teaches the one idea the whole game rests on: a group with no liberties comes off the board.

Which rules does this use?

Suicide is illegal, ko is positional superko — no move may recreate any earlier position, not just the previous one — and scoring is by area, the Chinese count: your stones plus the points they surround. Komi always carries a half point, so a game cannot be drawn.

Why did it stop and ask me to mark dead stones?

Because both players passed, and that ends the play rather than the game. What a group is worth at the end depends on whether it would have survived, and no program can answer that correctly in every position — seki and big-eye shapes have defeated every simple rule for as long as the game has been written down. So you decide. The Suggest button offers a cautious guess and you can change any of it.

How strong is the computer?

Strong enough to punish loose groups on 9×9 and 13×13, which is what it is tuned for. It plays out thousands of complete games in its head for every move — about eight thousand a second on 9×9 — and the level you choose is simply how many. On 19×19 the same effort is six times less search, which is why the full board here is for two people at one device rather than against the computer.

Where is my progress kept?

In this browser, and nowhere else. Each board size, game and level keeps its own record, because a 9×9 win and a 19×19 win are not comparable. Clearing your browser data, or using the button on this page, removes all of it permanently.

• GomokuFive in a row on the same kind of grid, and much quicker to learn.
• DominoesDraw and Block, two to four players, and hidden hands.
• ReversiSix opponents, an analysis board and solved endgame puzzles.