Reversi

What Is Reversi?

Reversi is a strategy game for two players, played with 64 discs on an 8x8 board. Each disc is dark on one side and light on the other. On your turn you place a disc so that it traps a line of your opponent's discs against one of yours, and every trapped disc flips to your colour. When neither player can move, the one with more discs wins.

Our board uses the modern starting position and rules, the same ones used by Othello, the trademarked version of the game: four discs start in the centre, Black moves first, and a player with no legal move passes. You play Black against the computer, pick Easy, Medium, or Hard, and every square where you can legally play is marked on the board.

How to Play Reversi

Place one disc a turn so that it traps at least one line of your opponent's discs, flip every disc you trap, and finish with more discs than your opponent. Black always makes the first move.

1. Set up the centre. Put white discs on d4 and e5 and black discs on e4 and d5, so each colour sits on a diagonal. The other 60 squares start empty.
2. Let Black move first. Black has four possible first moves, d3, c4, f5, and e6, and each one flips a single white disc.
3. Trap a line. Place a disc on an empty square so that one or more of your opponent's discs lie in a straight line between it and one of your own discs, with no gap. The line can run across, up and down, or diagonally.
4. Flip everything you trap. Turn every trapped disc to your colour, in every direction your new disc traps a line. You cannot leave any of them unflipped.
5. Pass only when you are stuck. If no empty square would flip a disc for you, you pass and your opponent moves again. If you have a legal move, you must play one.
6. Count the discs at the end. The game ends when neither player can move, usually because all 64 squares are full. The player with more discs of their colour wins; equal counts are a draw.

Those four openings are mirror images of each other, so it makes no difference which one Black picks. Later in the game a single disc can trap lines in several directions at once, and every one of them flips:

These are also the rules of Othello, the trademarked version of the game: the same starting position, the same first player, and the same passing rule. The differences between Reversi and Othello come down to the opening, the name, and how tournaments score a finished game.

Reversi Rules

A move is legal only if it flips at least one disc, and it must flip every disc it traps. The details below settle the cases that cause arguments over a real board.

What counts as trapping a line

The trapped discs must all be your opponent's, in one unbroken straight line from your new disc to another disc of your colour. An empty square breaks the line. So does one of your own discs: the line ends at the first disc of your colour, and you cannot reach past it to trap discs further along. One move can trap lines in up to eight directions, and each of them flips.

No chain reactions

Only discs trapped by the disc you just placed turn over. A disc that changed colour this turn does not trap anything by itself; from the next move on it is simply one of your discs.

Passing

You pass only when no empty square on the board would flip a disc for you. Your opponent then moves again, as many times in a row as it takes for you to have a legal move. On our board the pass is automatic, and a message tells you who passed.

When the game ends

The game ends the moment neither player can move. Usually that means all 64 squares are full, but not always: a player whose discs have all been flipped has nothing to trap with, and the other player has nothing left to trap, so the game stops with empty squares on the board.

We ran every possible sequence of up to nine moves through our game engine. None ends the game in fewer than nine, and all 228 games that end on move nine are wipeouts by Black. The score is always 13-0, because four starting discs plus nine placed makes 13.

Counting the score

Count the discs of each colour. More discs wins, and equal counts are a draw. Tournament rules add one step: when a game ends with squares still empty, those squares are counted for the winner, so a 40-20 finish with four empty squares is recorded as 44-20, and a drawn game is always scored 32-32. Our board counts only the discs on the board, so its final score shows exactly what you see. The winner is the same under both methods.

Reading the notation

Columns run a to h from left to right and rows 1 to 8 from top to bottom, which is upside down compared with chess. The corners are a1, h1, a8, and h8. Our move history uses the same system, so an entry such as B: d3 (+1) means Black played d3 and flipped one disc.

Play Reversi Against the Computer

You play Black and the computer plays White, so you always make the first move. Teal rings mark every square where you can legally play, the most recent move is shaded amber, and the disc counts above the board update after each move. When one side has no move, the game passes for it and says so.

Choose the computer's strength from the menu above the board. The game starts on Medium.

• Easy picks a random legal move. Use it to learn how lines trap and flip.
• Medium scores every move it has: points for each disc flipped, a large bonus for a corner, a small one for an edge, and a penalty for an X-square or C-square next to an empty corner.
• Hard also judges the position each move leaves and counts the replies you would have, so it also plays to keep you short of moves.

Open Move History under the board to read the game back move by move; each entry shows the square played and the number of discs it flipped. The board keeps no saved game, so reloading the page starts a new one.

Reversi Strategy: How to Win

Take the corners, stay off the squares that give corners away, and keep your opponent short of moves. The disc count matters only at the end. For the first two-thirds of the game the better plan is usually to flip few discs and cut down your opponent's options.

Take the corners

A disc on a1, h1, a8, or h8 can never be flipped, because no square exists beyond it from which to trap it. An unbroken row of your discs running along an edge from your corner is just as safe.

Stay off the X-squares

b2, g2, b7, and g7 are the most dangerous squares on the board while the corner beside them is empty. A disc there lets your opponent play into the corner along the diagonal as soon as they have a disc further down it.

Our computer on Medium gives a corner a 200-point bonus, far more than any other square can score, so expect it to take a corner the moment you offer one. In this position it answers g7 with h8.

Be careful with C-squares

The edge squares beside a corner are less dangerous than X-squares but still risky while the corner is empty, most of all when your disc stands there alone. A single black disc on b1 can be trapped from a1 as soon as White owns c1, and then White has the corner and the start of the top edge.

Play for moves, not discs

Count your legal moves and your opponent's, not the discs. The player with more moves can wait for a good one. The player with few can be forced onto an X-square or a C-square.

Those extra moves come from White's frontier: the white discs that touch an empty square. Every one of them gives Black a place to play. Keep your own frontier short by choosing moves that flip discs inside your group rather than on its outside edge. Players call a move that flips no frontier discs a quiet move, and it is often the best one on the board.

The same idea explains why grabbing discs early backfires. Every disc you own is one your opponent can play against, so a big early capture usually hands them more moves. Medium on our board scores every disc it flips, so it grabs big captures early; let it.

Count the empty squares at the end

If nobody passes and the board fills, White makes the last move of the game, and the discs flipped on the last move stay flipped. Near the end the empty squares split into separate regions. Play first into a region with an odd number of empty squares, so that you also get the last move there, and leave the even regions for your opponent to open. A region of three works out as you, them, you. A single pass anywhere swaps who gets the last move.

Reversi vs Othello

Othello is Reversi with a fixed start and a trademarked name. In the original Reversi, played in England from the 1880s, the board starts empty and the first four discs go into the centre squares in whatever pattern the players choose. Othello, launched in Japan in 1973, sets those four discs out in advance, White on d4 and e5 and Black on e4 and d5, and has Black move first.

Othello is a registered trademark; outside Japan the rights belong to the toy company MegaHouse. Our game uses the Othello starting position and rules under the name Reversi, and the trapping and flipping rules are the same in both.

One variant turns the goal around. In Anti-Reversi, also called Reversed Reversi, every rule stays the same but the player with fewer discs at the end wins. Our board plays the standard game only.

History of Reversi and Othello

Reversi was on sale in England in the 1880s. Two Englishmen, Lewis Waterman and John W. Mollett, each claimed to have invented it and called the other a fraud. The earliest reliable mention in print is in The Saturday Review of 21 August 1886.

The version played today came from Japan. In 1973 the toy company Tsukuda launched Goro Hasegawa's Othello, named after Shakespeare's play. It sold well at once, and Japan held its first national championship the same year. The first World Othello Championship followed in Tokyo in 1977 and was won by Hiroshi Inoue. In 1980 Jonathan Cerf of the United States became the first champion from outside Japan.

In August 1997 the program Logistello, written by Michael Buro, beat world champion Takeshi Murakami six games to none. In October 2023 Hiroki Takizawa, a researcher at Preferred Networks in Tokyo, posted a computer proof, as a preprint, that perfect play by both sides from the standard starting position ends in a draw. He calls it a weak solution: it settles the starting position and gives a strategy that reaches the draw, not the result of every position. Our board starts from that same position, so a perfect game here would also end level.

Reversi FAQ