Which Reversi boards are solved?

A game is 'solved' when the perfect result from the start is known. For Reversi, the answer depends entirely on board size - and the biggest news came in 2023. Here is the state of play.

Quick answer: The small boards are solved: on 4x4 the second player wins with perfect play, and on 6x6 the second player wins 20-16. The standard 8x8 game was weakly solved in 2023 by Hiroki Takizawa and proven to be a draw with perfect play, though that took an enormous computation. Larger boards like 10x10 are still far beyond exhaustive analysis.

The solved small boards

Because the number of possible positions grows explosively with board size, the small boards were searched by computer first. On the 4x4 board the second player wins with perfect play, and on the 6x6 board the second player wins by a score of 20 to 16 (solved by J. Feinstein in 1993). These results make the small boards excellent for training your calculation, because a whole game can be read to its end.

The 8x8 game: solved in 2023

The standard 8x8 Othello board has around 10^28 possible positions, so for decades it was assumed to be beyond reach. In 2023, however, Hiroki Takizawa weakly solved it by computer and proved that perfect play by both sides ends in a draw - confirming what strong programs had long suggested. It remains far too deep for any human to play perfectly. The even larger 10x10 board is still completely beyond current analysis, which is part of its open-ended appeal.

Try a solved board - play 6x6

Related questions

What is Reversi?

Reversi is a two-player abstract strategy game played on an 8x8 board with double-sided discs, one colour per player. On your turn you place a disc so it traps a straight line of your opponent's discs between two of yours, flipping them all to your colour. When the board fills up, whoever has more discs wins.

How do you win at Reversi?

Winning Reversi comes down to three ideas: grab the corners safely because corner discs can never be flipped; play for mobility by keeping many good moves for yourself and few for your opponent; and control parity so you make the last move in each region. Chasing raw disc count early is usually a mistake.

How long does a game of Reversi take?

A standard 8x8 Reversi game lasts at most 60 moves and usually takes three to six minutes. Smaller boards like 4x4 and 6x6 finish in a minute or two, while the larger 10x10 Grand Reversi board can run five to ten minutes or more.