Go · Weiqi · Baduk
Go is the oldest board game still played in its original form, and the rules fit on a postcard. If you have never played, start on 9 × 9 — a game takes about ten minutes and everything below still applies.
Every stone breathes through the empty points touching it — its liberties. This lone stone has four.
Fill the last liberty and the stone is captured and lifted off. White plays the green point to take it.
Playing into a point with no liberties is suicide and is not allowed — unless the move captures something first.
A group with two separate eyes (A and B) can never be captured — the opponent would have to fill both, and filling either one is suicide.
Black can capture the marked point — but White may not take straight back, or the two would repeat forever. White must play elsewhere first.
At the end, you score your stones plus the empty points only you surround. Black’s wall seals nine points in this corner.
When you both pass, the board switches to scoring. Groups that cannot escape are dead even though they are still sitting there, so click any such group to remove it — the board shows the effect on the score immediately. Press Confirm result when it looks right, or Keep playing if you would rather settle it on the board.
This game uses Chinese (area) scoring: your score is your stones on the board plus the empty points only you surround, and White adds komi for moving second. Area scoring is used here because it can be counted automatically without the two players having to agree on the fiddly cases first. One practical consequence: filling in your own territory at the end costs you nothing.
Go handles uneven players better than any other game. The weaker player takes Black and starts with 2–9 stones already on the board, and komi drops to half a point. Two grades of difference ≈ two stones.
Be straight about what it is: a heuristic practice opponent, not a real engine. Modern Go programs are neural networks weighing megabytes; that has no business loading on a small-business website. This one takes stones it can take, saves its own groups when they are down to one liberty, prefers the third and fourth lines early, answers near your last move, and never fills its own eyes. Beginner plays loosely and misses tactics, Casual plays them most of the time, Club plays them consistently. It is a genuine opponent for a new player and a warm-up for anyone else.
Host a room and share the 4-digit PIN, or tap the ET room. The front two in the queue play; everyone else watches the live board. Winner stays on, loser to the back of the line, colours swap each game. Everyone plays the host's board size, handicap and komi, and at scoring time the host marks the dead groups so both screens always show one agreed result.