How to play Y

Y is a connection game for two players on a triangular board. Black moves first. On your turn you place one stone on any empty point. Stones are never moved, never captured, and never removed. You win the moment one group of your stones touches all three sides of the board.

Black has won: a single connected chain of black stones reaches the red, the green and the blue side at once. The white stones are in the way but they are not in that chain, so they do not matter to it.

The two rules that catch people out

A corner counts for both of its sides. A stone in a corner is on two sides at the same time, which makes corners cheap and strong.

There are no draws, ever. Fill a Y board completely with black and white stones, any way you like, and exactly one colour will have connected all three sides — never both, never neither. So there is nothing to play for but the win, and the game always has one. (This is the theorem Y is famous for, and it holds on every board on this site, including the irregular Mudcrack ones.)

The pie rule

Moving first is an advantage, so the second player gets to take it away. After Black's first stone, White chooses: reply normally, or take that opening point as their own — colours effectively swap, and Black must answer on the board they built.

On this site the swap is a click: when it is White's first turn, White may click the black stone; it turns white and wears a π until Black has replied. The usual consequence is that Black opens with a point they would be content to hand over — the fairest point on the board rather than the strongest.

What to aim for

Reach in three directions, not one. A chain along one edge is easy to build and wins nothing; a group that grows toward all three sides is the whole game.
Prefer connections that cannot be cut. Two stones with two separate ways to join are already joined: if your opponent blocks one, you take the other. Playing in contact is often slower than leaving that gap.
Blocking is connecting. Every stone that stops their group reaching a side is also a stone building yours. There is no such thing as a purely defensive move.

Playing here

Ginny, the computer opponent on the board page, runs entirely in your browser. Nothing is sent anywhere unless you play a friend online.

Board: Board size, from a 9-point board up to 685 points. The rules are identical on all of them; the big ones are a different game in practice.
Black / White: Who plays each colour: you (Human) or Ginny, the trained network (AI). Set both to AI to watch it play itself. Swap exchanges the two, carrying each side's skill with it.
Skill (1–10): How strongly Ginny plays. Each step doubles the number of positions it may look at before moving and makes its choice less random: 1 plays on instinct alone, 10 looks at up to 4096 positions. Without a clock it also sets how long Ginny may think — about half a second at 5, about half a minute at 10.
Clock: Optional time control in the usual notation — 3 | 2 is three minutes each plus two seconds added after every move. It starts after Black's first stone, so the pie-rule decision is untimed. Under a clock Ginny spends a tenth of its remaining time on each move, plus the increment; its skill still limits how far it looks.
Play a friend: Starts a game on your board and clock and gives you a link to send to a friend; whoever opens it takes the other seat. Games other people have offered are listed too — click one to play it on its terms. No account, just a name your opponent sees. Resign ends it.
Undo: Takes back your last move, and Ginny's reply to it. Not available in an online game.

Something stranger

Mudcrack Y is the same game on a board of irregular cells that is generated fresh for every game — and the same network plays it, having never seen a board like it. How that works.