Learn Ultimate Tic-Tac-Toe

The board is a three-by-three grid of small tic-tac-toe boards — nine boards, eighty-one cells. X moves first. Winning a small board with three in a line claims it, and claiming three small boards in a line wins the whole game. That much is just tic-tac-toe twice over. The rule that makes it a game is the sending rule below, and it is the only one worth memorising.

• Eighty-one cells, but you can usually only reach nine of them.
• X always opens.

The cell you play in decides the board your opponent must play in next. Take the top-right cell of any board, and your opponent must play somewhere in the top-right board. Take the centre cell, and they are sent to the centre board. So you are never simply choosing the best square in front of you. Every move is two decisions at once: what it does for you here, and where it puts your opponent. Beginners lose because they only make the first decision. On Cognivo, hovering a cell outlines the board it would send your opponent to, so you can see the second decision before you commit to it.

• Hover a cell to see where it sends your opponent before you play it.
• A move that is good locally and sends your opponent somewhere useful is often worse than a mediocre move that sends them nowhere.

If you are sent to a board that is already finished — won by either side, or drawn — you are not stuck. You may play in any unfinished board instead. That is a free choice, and it is the most valuable thing you can be handed in this game. Which means the reverse is the thing to avoid: sending your opponent to a finished board hands them the run of the whole position. Late in a game, when several boards are decided, half the skill is counting which cells still send somewhere and which have quietly become free passes.

• Free choice is the strongest position in the game. Give it away as rarely as you can.
• As boards finish, more and more cells turn into free passes. Recount every turn.

Three claimed boards in a line — across, down, or diagonally — wins immediately. A small board with every cell filled and no three in a line is drawn, and a drawn board counts for neither side; it is simply out of play and sends anyone pointed at it somewhere else. The whole game is drawn when every board is decided and no side holds three in a line. Cognivo shows the running count of boards held by each side beside the board, because the score of a position this cluttered is genuinely hard to read at a glance.

• A drawn board is not neutral ground — it is a free pass for whoever gets sent there.
• Watch the board count, not the cell count. Cells are only a means to boards.

The commonest beginner mistake is treating each small board as its own game and playing it out to win. It costs games, because a small board is not the prize — it is one of nine cells in the only board that matters. So losing a small board on purpose is a normal move here. If giving up the bottom-left board keeps your opponent out of the centre for three turns, the trade is usually good. Ask what a move does to the big board first, and what it does to the small one second.

• Give up a small board without regret if the tempo is worth more.
• Ask what a move does to the big board before you ask what it does to the small one.

The centre board sits in four of the eight winning lines — more than any other — so it is the most valuable board to hold, exactly as the centre square is in ordinary tic-tac-toe. The centre cell of each board is valuable for a different and more interesting reason: playing it sends your opponent to the centre board. That makes centre cells the standard way to fight for the centre without playing there yourself, and it is why strong games so often circle the middle for the first dozen moves.

• Centre board: four winning lines. Corner boards: three. Edge boards: two.
• Playing a centre cell is a way of contesting the centre board without being in it.

The move that actually decides most games is the one creating two threats your opponent cannot answer together. It happens on both levels. On a small board it is ordinary tic-tac-toe: a fork with two ways to make three. On the big board it is the same shape one level up — reaching a position where you threaten to claim either of two boards that each complete a line. Because claiming a board takes several moves rather than one, a big-board double threat is something you build over many turns, and it is usually invisible to an opponent who is watching only the board in front of them.

• Count your opponent's threats before your own; one you cannot answer ends the game.
• A big-board fork takes several moves to build, so it is easy to miss and hard to stop late.

Strong play in this game is mostly about where your opponent is allowed to be. There are two ideas worth naming. The first is the send-back: play a cell that points at a board where every good reply also sends you somewhere you want. Chains of these are how good players keep an opponent shuffling between two harmless boards while they build elsewhere. The second is the poisoned board. If a board is nearly won by you, it is dangerous for your opponent to be sent there — they must spend their move blocking rather than developing. Deliberately leaving a board one move from completion, and repeatedly sending your opponent into it, is a way of playing several free moves in a row.

• Look for cells whose every plausible reply sends you somewhere good.
• A board you nearly own is a trap: your opponent has to spend a move there rather than building.