Reversi feels like a game about flipping as many discs as you can. It is not. The disc count swings wildly until the last few moves, and the player who grabs the most early often runs out of good moves later. These ideas win games in Reversi.
Corners are worth the most
A disc in a corner can never be flipped: there is no square beyond it to outflank it from. Discs built out from a corner along the edges become safe too.
Taking a corner is usually the strongest move on the board, and giving one away the most costly mistake.
Stay off the squares next to an empty corner
The square diagonally next to a corner, and the two beside it on the edges, are dangerous while the corner is empty. A disc there can give your opponent a line into the corner.
Play them only when the corner is already taken, or when the alternative is worse.
Mobility: keep your options, take theirs away
What matters in the middle game is how many legal moves each side has. Running short of moves forces you into bad squares, or to pass.
- Prefer quiet moves that flip few discs, especially discs in the middle of the board.
- Avoid frontier discs: discs that sit next to empty squares give your opponent places to play.
- A move that leaves your opponent only poor squares, like those next to corners, is often better than one that flips many discs.
Edges: useful, but not automatically good
Edge discs are harder to flip than middle ones, but a single edge disc can be outflanked along the edge. Build edges in solid runs that reach a corner, and be careful about leaving one empty square between your discs on an edge, which gives the opponent a way in.
The endgame: count and plan
With a dozen or so empty squares left, the game becomes a calculation. Look at each empty region of the board: having the last move in a region usually gains discs, so avoid moves that hand your opponent the final move in an area.
Remember passes: if your opponent has no legal move, you play again, which can let you take several squares in a row.
Beating each computer level
Easy looks one move ahead and plays a random legal move about a third of the time. Take the corners it gives away and avoid giving it any.
Medium looks three moves ahead and values the corners, and it plays perfectly once 6 or fewer squares are left. Win the middle game on mobility before then.
Hard looks 5 moves ahead and plays perfectly from 8 empty squares. You need a clear lead in corners and safe edges before the last 8 squares, because it will not miss anything after that.
