Tic-tac-toe: the never-lose strategy

Tic-tac-toe is a solved game: with correct play from both sides, every game is a draw, forever. That sounds like it kills the fun, but it does something better — it means there is a complete strategy, learnable in ten minutes, after which you will never lose another game. Here it is, with the reasoning attached. (Practise against our tic-tac-toe — its perfect mode plays exactly the strategy on this page, so a draw against it is a diploma.)

A solved game

"Solved" means the outcome under best play is proven — checkers is solved (draw), Connect Four is solved (first player wins), and tic-tac-toe is small enough that you can verify it by hand in an afternoon. The verdict: neither player can force a win against correct responses; every decided game contains a mistake, and the whole art is being the one who doesn't make it. First player (X) has the initiative — more chances to profit from errors — but no forced win.

The best first move

The corner is the strongest practical opening. The centre is the theoretically "best" square (it sits in 4 of the 8 winning lines), but against a corner opening there is exactly one non-losing reply: the centre. Every other response — edge, adjacent corner, anywhere — loses to correct play. Against a centre opening, any corner reply holds the draw, so a novice survives it more often by accident. Corner first punishes the most people.

The worst opening is an edge (middle of a side): it sits in only 2 winning lines and hands the initiative away.

The fork: the only way anyone wins

Nobody past age six loses to an unblocked line. Every real win comes from a fork — a move creating two winning threats at once, so the defender blocks one and dies to the other. The classic: X takes a corner, O replies with an edge (mistake), X takes the opposite corner, and now the centre — or another corner — gives X two lines through one square. Learn to see forks a move early, in both directions: your next move's fork is a win; your opponent's is the thing rule 5 below exists for.

The defensive rules

Perfect play is this priority list, checked top to bottom every turn:

• 1. Win: complete your own line if you can.
• 2. Block: if they threaten a line, kill it.
• 3. Fork: create a double threat if available.
• 4. Block their fork: if they can fork next turn, either occupy that square or — the expert version — make a threat of your own that forces them to respond elsewhere (just not a threat whose block creates their fork).
• 5. Centre, if free.
• 6. Opposite corner to an opponent's corner.
• 7. Any empty corner. 8. Any edge.

One trap deserves its own sentence, because it beats everyone once: X corner, O centre (correct!), X opposite corner — and O, feeling clever, takes a corner. That corner move loses: X's third corner forks. The correct O reply to the double-corner setup is an edge — the one position where the humble edge is the hero.

255,168 games, counted

The game tree is small enough to enumerate exactly: 255,168 possible games, of which X wins 131,184, O wins 77,904, and 46,080 draw. Two things worth reading out of the numbers: random play favours X heavily (the initiative is real), and draws are under a fifth of possible games even though they're 100% of correct ones — a neat, checkable illustration of how far random outcomes sit from optimal ones. Up to symmetry, the 9 openings collapse to 3: corner, edge, centre — which is why the strategy above fits on a page.

When you outgrow it

Which happens — a solved game with a one-page solution has a shelf life. The same "see the double threat" instinct scales up: Connect Four is the natural next game (solved, but the solution doesn't fit on a page), and Minesweeper or 2048 trade the opponent for logic and probability. But teach tic-tac-toe's priority list to a kid first — it's the smallest complete lesson in thinking one move ahead that exists.