Reversi — the free browser board game against a computer that improves

An 8×8 board, four discs in the middle, and one rule: place a disc so that it traps an unbroken line of your opponent's discs between it and another of your own, and every trapped disc flips to your colour. You are dark and you move first. When neither player can move, whoever owns more discs wins.

What turns one board game into a score chase is the opponent. Every game you win, it searches one move deeper — it starts out barely looking past its own move and ends up seeing six plies ahead, which is far enough to set traps you will not notice until the corner is already gone. Your score is your win streak: a loss or a draw ends the run. Free, no download, and the board scales to a phone.

How to play

• Tap a glowing pip to place a dark disc there. Those pips are every legal move you have — a move is only legal if it flips something.
• Discs flip in all eight directions at once. One well-placed disc can flip three lines in a single move.
• No legal move? You pass automatically and the board tells you so. If neither side can move, the game ends there — a full board is not required.
• Most discs at the end wins. Your score is consecutive wins; a loss or a draw ends the run.
• The computer's search depth rises with your streak, so your fifth opponent is a genuinely different player from your first.

Why corners win games and grabbing the most discs loses them

Start with the one square of the board that behaves differently from all the others. A disc in a corner can never be flipped. Flipping requires an unbroken line bracketed by two enemy discs, and a corner has no square beyond it on either side of any line running through it — so there is nothing to bracket it with. Every other disc on the board, including the ones you have held for twenty moves, is temporary. Corners are the only permanent property in Reversi, and they anchor whole edges: discs sitting in a line out from a corner you own become unflippable too, which is how a small stable region grows into a won endgame.

That single fact makes the two squares nearest each corner the worst on the board. The X-square — the one diagonally touching a corner — is poison, because playing there puts a disc on the exact diagonal your opponent needs to bracket in order to take the corner itself. The C-squares, the two edge squares beside a corner, leak the corner the same way along the edge. The computer you are playing scores the board from a weight table that reflects this directly: the corners carry the largest positive value, the X-squares the largest negative, and the C-squares are negative too. If it declines an obvious-looking move next to a corner, that is why.

Now the trap that catches nearly every new player: taking the most discs early is actively bad. Flipping a huge line feels like progress, but the discs you just converted are all still flippable, and every one of them is a square your opponent can now play next to. What actually decides the game is mobility — how many legal moves you have. A player with two legal moves is choosing between two bad ideas; a player with twelve can afford to wait. The strongest way to play the opening is almost the opposite of the instinct: flip as few discs as possible, keep your discs clustered near the centre, and leave your opponent surrounded by your colour so their options dry up. Do it well enough and they run out of safe moves entirely and are forced to hand you a corner — or to pass, which gives you two moves in a row.

This is why the opponent here does not simply count discs. Its evaluation adds up the positional weight of every disc, adds a term for the difference in legal moves between the two sides, and weighs the raw disc difference only lightly until the board is nearly full, when it finally starts to matter because there is no time left to flip anything back. Play against it with the same priorities — corners first, mobility second, disc count last until the very end — and you will start winning streaks instead of single games.

More games

More games that make you think: Four in a Row, Sudoku, or Minesweeper. Or take three in a row against a computer that turns perfect in Tic Tac Toe, or sow stones in Mancala. Or browse all games.