Reversi

How to play

Place a disc so that a straight line of your opponent's discs sits between the square you played and another of your discs. Every disc in that line flips to your colour, and lines run in all eight directions. You may only play a move that flips at least one disc, and if you have no such move your turn passes. The game ends when neither side can move, and whoever owns more discs wins. Legal moves are marked on the board while it is your turn.

Tips

• Corners can never be flipped, so they are worth more than any number of discs elsewhere.
• Avoid the squares diagonally next to a corner. Playing one usually hands your opponent the corner.
• Owning fewer discs in the early game is often good. Fewer discs means more moves available later.
• Count your legal moves, not your discs. A position where your opponent has almost no move plays itself.
• Edges are strong but only when solid. A lone edge disc with gaps beside it is easy to undermine.

Reversi and Othello

Reversi was invented in England in the 1880s, and the version most people know today is Othello, a 1971 Japanese refinement that fixed the starting position and became a worldwide competitive game. The rules here follow Othello: the four centre squares begin occupied in a diagonal pattern, black moves first, and a move must flip at least one disc.

The defining feature of the game is that the score means almost nothing until the end. Discs flip back and forth constantly, and a player who owns most of the board in the middle game often loses. What matters is the squares that cannot be flipped, which is corners, and the number of moves each side has available, which is mobility. The computer here weights exactly those things rather than counting discs, which is why it plays sensibly instead of greedily.

How strong players think

• Corners are permanent. A disc in a corner can never be flipped, and it anchors every disc along both edges from it.
• The squares diagonally adjacent to a corner are poison. Playing one usually hands the corner to your opponent.
• Fewer discs is often better in the midgame. Every disc you own is one your opponent can flank, and fewer discs usually means more moves.
• Count moves, not discs. A position where your opponent has almost no legal move is close to won even if they own more of the board.
• Parity in the endgame: the player who makes the last move in a region usually takes it. Leave an odd number of empty squares where you want to move last.

Common questions

Why do I keep losing when I have more discs?

Because discs flip back. Midgame disc count is a poor guide. Corners and the number of moves you have available decide most games.

What happens if I have no legal move?

Your turn passes and your opponent moves again. The game ends when neither side can move.

Which squares are dangerous?

The three squares touching each corner, and especially the diagonal one. Playing them tends to give the corner away.