What this game is
Reversi is a game where no disc is ever really yours. Everything on the board can change colour, and most of it changes more than once.
It looks calm from the outside: two players taking turns dropping discs onto a chequered board. It is really a game about space rather than about counting. The player who leads on discs in the middle of the game usually loses — his discs are exposed, easy to outflank, and his choices keep shrinking.
The last ten moves can turn 20:44 into 40:24. That is why a game of Reversi is worth watching to the end rather than abandoning halfway.
Rules
- The board is 8×8. You play the blue discs, the computer plays red, and you move first.
- Four discs start in the centre in a cross pattern: two of yours and two of the computer's, set diagonally.
- A move places one of your discs on an empty square so that between it and another disc of yours there is an unbroken line of enemy discs — horizontal, vertical or diagonal — with no gaps and no empty squares.
- Every enemy disc outflanked that way is flipped and becomes yours. If a move outflanks lines in several directions at once, all of them flip.
- A move that flips nothing is illegal. The squares you may play are marked with a faint dot, so you never have to trace the lines by eye.
- A player with no legal move passes, and the opponent moves again. The program says so in a small dialog.
- The game ends when neither side can move. Usually the board is full by then, but not always: sometimes nobody has a move with several squares still empty.
- The player with more discs wins. An equal count is a draw, and a draw is not added to either tally in the strip under the board.
How to play better
- Do not chase discs. In the opening and middlegame it pays to own fewer of them: the tighter your group, the fewer squares your opponent has to play at all.
- Count moves, not discs. A good position is one where you have ten options and your opponent has one or two, both bad. A player with no move has to pass, and you get two turns in a row.
- Corners never flip: a corner disc has no square behind it on any line, so nothing can outflank it. A corner you take is an anchor for the rest of the game.
- Which is why you should avoid the squares next to an empty corner, especially the diagonal one (b2, g2, b7, g7). A disc there is usually a direct invitation into the corner.
- Stable discs — the ones that can no longer be flipped — grow out of the corners along the edges. Build your position from a corner outwards, not from the centre.
- Leading on discs ten moves from the end means nothing. Look at who will get the last empty squares: that is where the result is decided.
A bit of history
In 1883 two men in England put the game on sale at once: Lewis Waterman and John W. Mollett. Each claimed to be the inventor and called the other a fraud, and the question has never been settled. Dispute or not, Reversi became one of the hits of Victorian England.
The game was born a second time in Japan. In 1971 Goro Hasegawa settled the rules into their present form and registered them as Othello, after Shakespeare's tragedy, where black and white keep changing places. Under that name it spread worldwide and became one of the best-selling board games ever made.
This version follows the applet that ran on this site from 2005: the same cyan panel, the same old-fashioned menu bar, the same grey flash before the computer moves. The Easy level is its original algorithm, carried over untouched.
About the levels
Easy is the 2005 algorithm. It walks through its legal moves and takes whichever one flips the most discs right now. It sees neither corners nor your reply: giving up a corner for four extra discs is a perfectly normal move as far as it is concerned.
Hard searches four plies ahead with alpha-beta pruning and handles passes correctly. What it still cannot do is judge a position: it counts discs and nothing else, so a corner is worth exactly one disc to it, the same as any square in the middle.
Expert goes from three to seven plies deep, as far as it gets within roughly a third of a second, and scores positions with the classic weight table: a corner is worth 100, the diagonal square beside it minus 50. On top of that it adds the difference in available moves, so it plays for mobility. Once twelve or fewer squares are empty it switches to a plain disc count and plays the endgame almost exactly. You can beat it, but not by being greedy.