Reversi

Strategy

The rules, precisely

Reversi is played on an 8×8 board with 64 identical discs, dark on one side and light on the other. The game starts with four discs in the centre: white on d4 and e5, black on d5 and e4. Black moves first. A move places one disc of your colour on an empty square so that at least one straight line of your opponent's discs — horizontal, vertical or diagonal, with no gaps — lies between the new disc and another disc of yours. Every disc in every such line is flipped to your colour. A single move can flip lines in several directions at once, but only lines that are closed at both ends by your own discs. If you have no legal move you must pass, and while you have one you may not. The game ends when neither player can move, usually because the board is full, and the player with more discs wins.

Corners are forever

A disc in a corner can never be flipped, because there is no square beyond it to outflank it from. That makes the corners the most valuable squares on the board, and once a corner is yours, the discs you build out along the edges from it often become unflippable too. Most games between experienced players are decided by who takes more corners — or, more precisely, by who was forced to give them away.

Stay off the X-squares

The four squares diagonally next to the corners — b2, g2, b7 and g7 — are called X-squares, and playing on one while its corner is still empty is the classic beginner's mistake: it hands your opponent a diagonal line straight into the corner. The edge squares beside each corner, the C-squares, carry a smaller version of the same risk. Once a corner has been taken, the squares around it lose their danger, and the computer here stops avoiding them for exactly that reason.

Mobility beats material

It feels natural to flip as many discs as you can, and in the opening it is usually wrong. A player with lots of discs tends to have few legal moves — there are not many of the opponent's discs left to outflank — and a player with no good moves ends up forced onto X-squares and giving corners away. Strong players try to keep the number of moves open to them high and their opponent's low, often while holding fewer discs, and only start caring about the count near the end. The computer's evaluation weighs exactly that: corners first, then mobility, and the plain disc count only in the endgame.

Frontiers and quiet moves

A disc next to an empty square is a frontier disc, and every frontier disc is a potential move for your opponent. The best moves are often quiet ones that flip discs in the interior, surrounded on all sides, so they open up nothing new. When you have a choice, prefer the move that flips fewer discs and exposes fewer new edges. That is how an opponent is slowly squeezed out of safe moves until the only ones left give up a corner.

Parity in the endgame

Towards the end the board breaks up into small empty regions, and you generally want to be the player who makes the last move in each one, because that move cannot be answered there. Heading into regions with an even number of empty squares and leaving your opponent the odd ones is called parity, and it settles many close finishes. On the harder levels the computer does not rely on the rule of thumb: once few enough squares are empty, it counts out every possible ending.

A short history of Reversi

Reversi was devised in England in the 1880s, where two men, Lewis Waterman and John W. Mollett, each claimed to have invented it, and in its early form players could place their first discs wherever they liked. The modern game with the fixed four-disc start was popularised in Japan in the 1970s and sold under the trademark Othello, and a world championship for it has been held since 1977. A version of Reversi was also one of the games bundled with early releases of Microsoft Windows, which taught the game to a generation of office workers.

Is Reversi solved?

In 2023 researcher Hiroki Takizawa reported a computer proof that the standard 8×8 game is a draw with perfect play from both sides. Smaller boards were settled much earlier: 6×6 was solved in 1993, and perfect play there ends 20–16 to White, the second player. The size of the problem explains the wait — the 8×8 board is estimated to allow up to around 10²⁸ legal positions — and it is why the computer here searches from the position in front of it rather than looking anything up.

What the difficulty levels actually do

Every level runs the same alpha-beta search over the real rules. Easy looks one move ahead and plays a random legal move about four times in ten, so it gives corners away while you learn. Medium looks three moves ahead, slips occasionally, and counts the last 6 empty squares out exactly. Hard looks up to five moves ahead and counts out the last 10. Expert searches as deep as it can in a little under a second and tries to solve the final 14 squares outright. Until the count takes over, positions are judged the way strong players judge them — corners, the danger of X- and C-squares, mobility and frontier discs — rather than by who has more discs. The search is time-capped, so on a slow phone it plays a slightly shallower move rather than freezing the page.

Two players on one device

Switch the opponent to 2 players and pass the device between turns. Legal moves are marked for whoever is to play, passes are announced so nobody misses a turn, and undo works exactly as it does against the computer. Results go into a separate session tally rather than your score against the computer.

Playing without a mouse

The arrow keys move around the board, Enter or Space plays on the focused square, U undoes, H shows a hint and N starts a new game. Every square is a real button with a label — its name, what is on it and, for a legal move, how many discs it would flip — and the turn line is a live region, so passes and results are announced as well as shown.

Nothing here leaves your device

The game runs entirely in your browser. There is no account and no server keeping your games, and the computer's search runs on your own device, which is also why it answers instantly. Your score, your settings and the game in progress are kept in your browser's local storage; clearing your browser data resets them, and nothing is stored anywhere else. For another long-think strategy game try Checkers, or Connect Four for something faster.