The rules, in about a minute
Players take turns placing a stone on any empty intersection. Black moves first. A stone or a connected group of stones is captured and removed when it has no liberties — no empty points directly adjacent to it. You may not play a stone that would leave your own group with no liberties, unless that same move captures something. When both players pass, the game is over.
Ko stops an endless loop. If you capture a single stone and your capturing stone is left with exactly one liberty, your opponent cannot immediately take it back — they must play elsewhere first. On this board that banned point is marked with a cross, along with every other point you cannot legally play. Every other site just ignores the click and leaves a beginner wondering what they did wrong.
Area scoring, so nobody argues
At the end, your score is your stones on the board plus every empty point that only your stones touch. White adds a komi of 7.5 for moving second, and the half point means a draw cannot happen.
The alternative — territory scoring — requires both players to agree which groups are dead before the game can be counted, and disagreement about exactly that is a well-attested complaint about playing Go online — reviewers report being “surprised by their scoring results”, and dead-stone disputes have decided games between professionals playing under different national rules. Area scoring needs no such agreement. Two passes and the number is the number.
It also lets us show you the score for the whole game, in the panel beside the board, rather than revealing it at the end. Beginners abandon Go because they genuinely cannot tell who is ahead, and a running area count is the most honest answer available.
Why 9×9, and how we know the rules are right
A 9×9 game takes minutes and uses every rule the full board uses. We ship it because a 19×19 opponent worth playing is a research problem, and one size done honestly beats two done badly.
Go has no perft tradition, but it has something better for a rules engine: John Tromp's published counts of legal positions. A position is legal exactly when every group has a liberty, so counting them by brute force over a small board tests the group-and- liberty machinery — the only genuinely hard part — against a number computed independently. Our engine matches all of them: 1 on 1×1, 57 on 2×2, 12,675 on 3×3, and 24,318,165 on 4×4 across all 43 million colourings.
The 9×9 figure is 103,919,148,791,293,834,318,983,090,438,798,793,469 — about 1.04 × 10³⁸ — and no amount of brute force reaches it. We check what can actually be computed and state the ceiling rather than implying the larger number was verified. What the small boards prove is that groups, liberties and edge handling are correct, and those do not change with size.
The honest limitation
The computer opponent is weak. It avoids filling its own eyes — the classic way a naive Go bot kills itself — and it prefers captures and escapes, but it does not read ahead and it has no real territory sense. In self-play it produces lopsided games. It is a fine first opponent while you are learning the rules and it will not satisfy anyone who already plays.
We are saying this on the page rather than in a footnote because most people searching for this game are specifically looking for a computer opponent, and sending them away disappointed after twenty minutes would be worse than telling them now.