Why the disc count misleads
Reversi has huge swings. A row of your discs can turn over in a single move, and the score halfway through says little about who is winning. What counts is position: which squares you hold, how many moves you have, and who gets the last say in each part of the board. It is common for the player with fewer discs in the middle game to win comfortably.
Corners, X-squares and C-squares
A corner disc can never be flipped, because there is nothing beyond it to trap it against. The squares beside a corner are where corners are lost:
| Corner | C | A |
| C | X | · |
| A | · | · |
- X-squares sit diagonally next to a corner. Play one while the corner is empty and your opponent can often take the corner by trapping along the diagonal. Treat them as off-limits unless you have checked the corner can't be taken.
- C-squares sit beside a corner along the edge. They are less dangerous, but a disc there can let your opponent into the corner along that edge. Play one when the edge is settled or you can see the corner is safe.
- A-squares, two along from the corner, are usually sound edge moves.
Once a corner is filled, the squares beside it can't give it away any more.
Mobility: play for moves, not discs
Mobility is the number of moves you have. In the middle game, keep yours high and your opponent's low, until they are forced to play somewhere bad, such as an X-square, or to give up an edge.
- Make quiet moves. Choose moves that flip few discs, ideally discs inside your group rather than on its outside.
- Avoid frontier discs. A disc next to an empty square gives your opponent something to play against. The fewer of your discs touch empty squares, the fewer moves they have.
- Don't build walls. A long line of your discs facing empty space hands your opponent a row of moves; a compact group in the middle gives them few.
- Count the dots. Suvi marks every square you can play with a dot. Before you commit, picture how many dots your opponent will see after your move. Fewer is better.
Edges and stable discs
An edge disc can only be flipped along the edge, so edges are strong, but only once they can't be attacked. A lone disc on an empty edge invites your opponent to play beside it. Beware the wedge: a single empty square on the edge between two of your discs. Your opponent can often drop a disc into it that you can't flip back along the edge, and it often becomes a route to a corner. Try not to leave one-square gaps in your edges.
Discs that can never be flipped again are called stable: corners, filled edges, and discs packed against them. By the end it is stable discs, not the middle-game count, that decide the score.
Parity: getting the last move
The game starts with 60 empty squares. If nobody has to pass, the first player makes the odd-numbered moves and the second player makes the 60th, the last one. The last move in any part of the board is valuable, because your opponent gets no reply there.
Late on, the empty squares split into separate regions. The rule of thumb: play into regions with an odd number of empty squares, so that you will also play their last square, and leave even-sized regions alone. A pass changes who gets the last move, so a move that leaves your opponent with nothing to play can turn the whole ending round.
One Suvi detail: the game ends as soon as neither player can move, and only the discs on the board are counted. Empty squares go to nobody, so a game can be drawn even with gaps left.
Playing Suvi's computer
- Easy more often than not grabs the move that flips the most discs. Let it have plenty early and it will build walls and walk onto X-squares for you.
- Medium knows which squares are valuable and counts mobility, but looks only a few moves ahead. Quiet moves that cut its choices work well.
- Hard searches deeply, and once fourteen squares are left it usually works the ending out exactly. To beat it, you need a clear lead in position before then: more stable discs and more moves.
Undo takes back your move and the computer's reply, so you can test a plan; a hint shows the square Hard would choose.
If you only remember three things
- Corners first, and stay off the X-squares while a corner is empty.
- Fewer discs and more moves beats more discs and fewer moves.
- At the end, play into odd-sized regions.
Ready to try it? Play Reversi against Medium and see if you can win while trailing on discs until the last ten moves. For another long-think board game, try Gomoku.
Questions
- What are X-squares and C-squares in reversi?
- X-squares are the four squares diagonally next to the corners; C-squares are the eight squares next to a corner along an edge. Playing either while the corner is empty can hand the corner to your opponent, the X-square especially.
- Should I flip as many discs as possible?
- Usually not before the endgame. Big flips give your opponent more discs to play against and more moves. Quiet moves that flip a few inside discs keep your options open and limit theirs.
- What is parity in reversi?
- It is the idea of getting the last move. With no passes the second player makes the final move of the game, and in each separate region at the end, whoever plays first into an odd-sized region usually gets its last square too.
- What happens if the game ends before the board is full?
- On Suvi the game stops once neither player has a legal move, and the discs on the board are counted. Empty squares are not given to either player.
- Is the corner always the best move?
- Nearly always, but not automatically. A corner that you can take later, because your opponent can't reach it, can wait while you make a more urgent move elsewhere.
Play it now
Reversi is free on Suvi, on your phone or computer, no download. Also: Reversi vs Computer · 2 Player Reversi · Reversi With a Friend Online.