Reversi (Othello)
Play Reversi against a search-based computer at three depths, or against someone on the same device — with why holding corners beats holding discs.
Corners are the whole game
A corner can never be flipped once taken — it is the only permanent territory on the board, and everything else is on loan. The squares diagonally inside the corners are correspondingly the worst, because playing one usually hands the corner over, and the corner then eats the edge behind it.
Which is why being ahead on discs in the midgame is often a losing position. What matters is how many moves you still have compared with your opponent, and a big pile of discs tends to mean fewer. The computer scores mobility explicitly for that reason.
Things worth knowing
- A move is only legal if it flanks at least one opposing disc. There is no such thing in Reversi as a placement that captures nothing, which is why the legal-move list is so much shorter than the empty squares.
- Corners can never be flipped. That single fact is most of the strategy — a corner is permanent territory and everything else is on loan.
- The squares diagonally inside the corners are the worst on the board. Playing one usually hands your opponent the corner, and the corner then eats the edge.
- Being ahead on discs in the midgame is frequently a losing position. What matters is how many moves you have left compared with your opponent, and a big pile of discs tends to mean fewer.
- If you cannot move you pass, and play continues. The game only ends when NEITHER side has a legal move — which can happen with the board not full.
How to use
- Place a disc so it traps opponent discs between yours.
- Trapped discs flip to your colour.
- Take corners when you can and avoid the squares beside them.
- The player with more discs at the end wins.
Frequently asked questions
Why are corners so valuable?
Because a disc in a corner can never be flipped — there is no square beyond it to trap it from. Corners are the only permanently stable squares on the board, and they anchor stability along the edges leading away from them. Almost all serious Reversi strategy follows from this one fact.
Which squares should I avoid?
The squares diagonally adjacent to corners, sometimes called X-squares, and to a lesser extent the ones directly beside corners. Occupying them typically hands your opponent the corner, and losing a corner costs far more than the discs you gained taking the square.
Why is having more discs mid-game a bad sign?
Because it usually means you have more discs that can be flipped and fewer moves available. Strong play often involves deliberately holding very few discs in the middle game, restricting your opponent's options until they are forced into a move that gives you a corner.
What is mobility?
The number of legal moves available to you. Restricting your opponent's mobility until they must play a bad move is the central strategic idea, and it is why disc count means so little before the endgame. A player with two legal moves is usually losing regardless of the score.
Is Reversi the same as Othello?
Effectively the same game with different origins. Reversi dates to 1883 in England, with a disputed claim of invention between two men. Othello is the 1971 Japanese commercial version by Goro Hasegawa, which fixed the opening position and the board size — those rules are what almost everyone now plays.
Has the game been solved?
Not on the standard 8 by 8 board, though a 2023 result showed that perfect play leads to a draw with very high confidence. The 4 by 4 and 6 by 6 versions are fully solved, and computers have comfortably exceeded human play since the 1990s.
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