Reversi

Rules

The game starts with two light and two dark discs placed diagonally in the four central squares. Dark moves first. Each move places one disc so that one or more straight lines of enemy discs, in any of the eight directions, are enclosed between the new disc and another disc of yours. All enclosed discs flip.

If you have no such move you must pass, and the turn returns to your opponent. When neither player can move, usually because the board is full, the player with more discs wins; empty squares are counted for the winner.

Ideas that matter more than disc count

• Corners can never be flipped. A corner disc is permanent and often anchors a whole edge.
• The squares diagonally next to an empty corner are dangerous: playing there usually hands the corner to your opponent. The edge squares next to a corner carry a smaller version of the same risk.
• Mobility wins the middle game. Try to keep many moves for yourself and few for your opponent, even if that means having fewer discs for a while.
• Prefer quiet moves that flip few discs and do not touch empty squares. Discs on the border with empty squares give your opponent new moves.
• In the endgame, count regions. Try to make the last move in each enclosed region of empty squares.

Typical beginner errors

• Flipping as many discs as possible every move. It feels strong and usually loses.
• Taking an edge square without checking whether it gives the opponent a way to a corner.
• Forgetting that a pass is possible. Leaving your opponent without a move can be a weapon.

The computer at this table

At Champion strength the computer solves the last part of the game exactly once few empty squares remain, so a lead you carry into the final moves must be real.