Place a disc so it traps a line of your opponent's discs between the one you just placed and another of yours — every disc in that line flips to your colour. You can only play a move that flips at least one. The board fills up over the game, and whoever owns the most discs at the end wins. It looks like you're just covering squares; you're actually fighting over a few critical ones.
Play the AI across seven difficulty levels, with a fresh daily challenge each midnight. The challenge link lets a friend take on the same starting position.
Corners are forever — a corner disc can never be flipped, which is why strong play often means taking fewer discs early and refusing the edges until they are safe. Mobility beats material for most of the game. Seven AI levels climb from Beginner to Cyborg, the daily position is shared by everyone, and Go 9×9 is the deeper end of the same territorial pool.
I played Reversi with my brothers growing up, and they beat me. I say this begrudgingly, and I want the record to show two things clearly: we stopped playing, and I am still owed my revenge. Some games you build for the catalog. This one I built with a score to settle, and every design decision inside it is flavoured by the specific experience of losing, repeatedly, to people who clearly knew something I didn't.
What they knew — what I eventually learned the hard way and then encoded straight into this AI — is that mid-game material is a trap. The evaluation function here weights mobility and corner control over raw disc count, deliberately and heavily. A position where you hold fewer discs but more legal moves is usually the stronger position, because Reversi is a game of forcing: the player with options squeezes the player without them, and the final twenty moves flip entire walls of discs in ways the mid-game count never predicted. An AI tuned to hoard discs plays exactly the way I played against my brothers all those years ago. It feels ahead the whole game, and then it loses the ending in slow motion.
Which is the beginner mistake in one sentence: grabbing edges and discs early. Edges look safe — they're only flankable from one direction — and big captures feel like winning. But the squares that matter are the corners, the only cells on the board that can never be flipped once taken, and the deadliest squares in the game are their neighbours: the X-squares diagonal to each corner and the C-squares directly beside them. Occupy an X- or C-square next to an empty corner and you've usually handed your opponent that corner — a permanent, unflippable anchor from which they will re-flip everything you were so proud of collecting. The eager edge-grab is very often the exact move that loses the game thirty moves later, and nobody at the table connects the two events.
So play the game my brothers played against me. Count your legal moves and your opponent's — that difference is the real score for most of the game. Starve their mobility, keep your discs compact and central early, treat X- and C-squares as radioactive until their corner is resolved, and fight for corners like they're the only points on the board. Because effectively, they are. Disc count matters at exactly one moment: the end.
Seven AI levels, the top ones tuned with everything my losses taught me. The daily board lands at midnight, identical for everyone. As for my brothers — the rematch is coming, and this time I've had years of quiet practice against an opponent I built myself.
The counterintuitive truth of Reversi: having the most discs in the middle of the game is often bad. Discs flip constantly, so a big lead early means almost nothing — and grabbing lots of pieces usually just hands your opponent good moves. Early on I try to flip fewer discs, not more, and keep my options narrow for them.
What actually matters is corners. A disc in a corner can never be flipped — it's permanent — so corners are the real prize, and the squares right next to a corner are traps, because playing them often gives your opponent the corner. So my whole game is shaped around this: avoid the squares next to corners, force my opponent into them, and take the corners when they open. Mobility and corners win Reversi. Raw disc count in the middle is a mirage.
Reversi is the game that teaches you to ignore the scoreboard and play the position — being "ahead" mid-game is often a trap, and learning to feel that is a genuine shift in how you think. If you've ever felt that you fell behind early in something and the result was already decided — Reversi is the game that argues back. A board where you're trailing for forty moves can flip on the last few, because position matters more than the score along the way. It's not lost until the last move. Learning to feel that, to keep playing the position instead of the scoreboard, is the kind of patience that pays off well beyond a game.
You stop chasing the move that looks best right now and start asking what it costs you later. That's hard, and it's exactly the kind of thinking that transfers — to planning, to money, to any decision where the obvious win sets up a bigger loss. The AI levels climb to force it on you. No download, no sign-up — play in your browser on any device. If you like the long view, try Dots & Boxes.