Why the disc count lies
Reversi is a game where the score on the board means little until the very end. Discs change colour all game long, and a big lead in the middle often turns into a loss. Each move must flip at least one disc, so the player with fewer good options is the one who gets pushed around. Strong players care less about how many discs they have and more about which squares they hold and how many moves they have left.
On this board you play black and always move first.
Corners, edges and the squares to avoid
The four corners can never be flipped. A disc there is yours for good, and it lets you build stable rows along the edges beside it. So the main question all game is who gets the corners.
- The diagonal square next to each corner is the most dangerous square on the board. Play there early and you often open a diagonal that hands your opponent the corner.
- The two edge squares beside each corner carry a smaller risk of the same kind. Take them only when the corner is already settled or the computer can't reach it.
- Edges in general are fine once you understand them. A disc on an edge can only be flipped along that edge, so edge discs are harder to lose than centre discs.
Fewer discs, more moves
The second big idea is mobility: how many legal moves each side has. The faded discs on the board show your choices on every turn. Count them. Then think about how many you are about to give the computer.
- Make quiet moves. Prefer moves that flip few discs, and discs in the inside of the board rather than on its outer layer. Flipping a wall of discs on the outside gives your opponent new places to play against.
- Stay inside. Discs that touch empty squares are called frontier discs. The more you have, the more targets you offer. Keep your discs compact and let the computer spread out.
- Leave the computer short of moves. When it only has a few legal squares, it may be forced to play next to a corner. That's when you take the corner.
Here is a small example. Early in the game you see two options. One flips four white discs along the top edge of the central group and leaves a long black line facing several empty squares. The other flips a single disc tucked inside your group. The first looks greedy and strong. The second leaves the computer with fewer moves next turn and gives it nothing new to flank. In most positions the single quiet flip is the better choice.
Errors that cost games
- Grabbing discs early. A big flip in the opening usually means a big frontier. Leave the counting for the end.
- Playing next to an empty corner because nothing else looks good. Count the faded discs before you move. If every option is bad, choose the one that gives the least away.
- Forgetting parity near the end. When only a few empty squares remain, try to make the last move in each empty region. The last move can't be answered, so it keeps its flips.
Training with the three levels
Easy looks only one move ahead and mixes up its choices. It will often step onto the squares next to a corner, so it's the place to practise spotting those mistakes and punishing them. Medium plans three moves deep. It won't give corners away cheaply, so the lesson there is mobility: keep your moves quiet and see if it runs short of options. Hard reads five moves deep and plays the closing moves for an exact disc count. Against it, reaching the last part of the game with corners and more safe moves than the computer is the only reliable plan.
The Hint button rings the move a Medium-strength computer would choose. Use it after you've picked a move, not before. When the hint differs from your choice, work out why. That's the fastest way to learn what the program sees.
