Disc Sandwich

Rules and how to play

The board has 8 squares on each side, 64 in total, and each disc is black on one side and white on the other. At the start, 2 black and 2 white discs are placed diagonally on the 4 center squares. Black moves first and White second, and the players take turns placing one disc at a time. This page names squares by column a–h (left to right) and row 1–8 (top to bottom). In the starting position, d4 and e5 are white, and e4 and d5 are black.^[10][11]

On your turn, place one of your discs on an empty square. You can place it only on a square where the disc you place and one of your discs already on the board outflank your opponent’s discs in a straight line, vertically, horizontally, or diagonally. Flip all of the outflanked discs to your color.^[10][11]

A line is outflanked when your opponent’s discs continue without a gap from the square next to the disc you placed, and the first square beyond them has one of your discs. A line with an empty square in it is not flipped. Nothing beyond the first of your own discs counts, so you never flip discs by jumping over one of your own. If you outflank discs in 2 or more directions, you flip the discs in all of those directions and cannot leave some of them unflipped. Flipped discs do not cause any further discs to flip.^[1][11]

If there is no square where you can outflank your opponent’s discs, you pass and your opponent plays again. You cannot pass if you have even one possible move. Passes may happen any number of times in a row. On this page, passes are made automatically.^[10][11][12]

The game ends when neither player has a possible move. Besides when the board is completely filled, the game also ends when all discs of one color are gone from the board, or when neither player can play even though empty squares remain. The player with more discs on the board wins, and if the numbers are equal, the game is a draw.^[2][10][11]

In the result, this page shows the numbers of discs on the board as they are, and adds the number of empty squares if any remain. In official tournament games, empty squares are counted for the winner, and a draw is recorded as 32 to 32. Wins, losses, and draws are the same with either counting method.^[12][14]

Controls on this page

Choose how to play in the panel to the right of the board (below it on narrow screens). With the Easy, Medium, and Hard buttons under “Play vs computer”, you play first (Black). To play second (White), press “Play second (White) · Medium” below them. With “2 players on this device”, 2 people take turns playing Black and White on one device.

Press “Play a friend (send a link)” to create a room and show a link. Copy it with “Copy link” and send it to your opponent (on phones and some other devices, you can also use “Share”). When your opponent opens the link and presses “Join”, the game starts. The host plays first (Black), and the player who joined plays second (White). After the game ends, press “Rematch (swap sides)” to start the next game. If your opponent stays disconnected for 2 minutes, you can press “Continue vs computer” to finish the game against the computer.

On the 3D board, when you move the mouse over a square where you can play, a faint disc appears there, and the discs that would flip are circled in yellow. Click to place the disc. On phones and other touch devices, the first tap shows the same preview, and tapping the same square again places the disc. Nothing happens when you press a square where you cannot play.

Drag the board to rotate the view, and use the mouse wheel or 2 fingers to zoom in and out. Switch the view with the “Angled” and “Top-down” buttons at the top right of the board. Columns a–h are marked on the near edge of the frame, and rows 1–8 on the left edge. When you play second, the board is not rotated and is shown facing the same way.

Squares where you can play are marked with a small dot. To hide the dots, clear the “Show possible moves” checkbox in the panel to the right of the board (below it on narrow screens). This setting is saved in your browser, so the board looks the same the next time you open the page. It does not affect your opponent’s screen in a game with a friend. You can also play with the “Button controls” grid in the same panel. In the grid, row 1 is at the top and column a is at the left, and only squares where you can play can be pressed. With a keyboard, move between squares with the arrow keys and press Enter to place a disc.

When you place a disc, the outflanked discs flip in order, starting with the ones closest to the disc you placed. If your operating system is set to reduce motion, they flip at once without animation. The numbers of black and white discs are shown at the bottom left of the board. The disc placed last has an orange ring, and the last move (for example, Black f5) and the number of discs it flipped are shown below the “Button controls” grid. When a player passes, a banner such as “White passes (no possible moves)” appears for a short time next to the disc counts at the bottom left of the board, and a note also stays below the grid until the player who passed plays next.

In games against the computer and in “2 players on this device”, you can take back moves with “Undo move”. Against the computer, this goes back to just before your last move and also takes back the computer’s moves after it. If you passed and the computer played more than once in a row, those moves are taken back together. “Play again” starts a new game, and “Resign” ends the game by conceding. Press “Change mode” to return to the screen where you choose how to play.

Strategy tips

• Take the corners (a1, h1, a8, and h8). A disc in a corner cannot be outflanked from both sides in any direction, so once placed, it is never flipped for the rest of the game. Your discs that continue without a gap from a corner along an edge can no longer be flipped either.
• While a corner is still empty, avoid playing on the square diagonally next to it (b2, g2, b7, and g7). If you have a disc there, it becomes easier for your opponent to outflank it and play in the corner. For the same reason, check before you play on the edge squares next to a corner (such as b1 and a2).
• In the opening and middle game, think about reducing the number of squares where your opponent can play rather than about flipping many discs. Flipping many discs increases the number of your discs next to empty squares, which gives your opponent more squares where they can outflank those discs and play.
• If you play so that the only squares left for your opponent are ones they do not want to play, such as the squares diagonally next to the corners, your opponent has no choice but to play there. If you leave your opponent no possible moves at all, your opponent passes and you play again.
• In the endgame, when the empty squares are split into several regions, play first in a region with an odd number of empty squares. If you both keep taking turns in that region, you play its last square, and your opponent cannot play back in that region.

Origin and history

The game of outflanking and flipping discs on a board was devised in Britain in the late 19th century and went on sale as a product in 1888 under the name reversi. The British weekly The Saturday Review introduced the game in its issue of August 21, 1886, under the title “Reverses”. The article says that the game uses a Checkers (Draughts) board and 64 discs, black (or red) on one side and white on the other, that it can be learned in 5 minutes, and that a game takes about 15 minutes. It also says that taking the corners is best and that the 3 squares next to a corner should usually be avoided, although there are exceptions.^[1][5][6]

Sources differ on who invented it. “Reversi and Go Bang”, a guide published in 1890 with the authorization of Lewis Waterman, describes the game it explains as the one invented by Waterman and published by Jaques & Son. It also says that there is a game called “Annexation, or Reversi” that is very similar in appearance and name, but that it differs in the quality of its rules and in how they are written. Meanwhile, John W. Mollett claimed that he had invented “Annexation”, played on a cross-shaped board, in 1870. Michael Keller says that Waterman’s claim is supported by newspaper articles and manuals of the time.^[2][3]

The rules in the 1890 guide differ from today’s rules in 2 ways. The game did not start with the first 4 discs already placed; instead, Black and White took turns placing them anywhere on the 4 center squares. Also, the 64 discs were divided into 32 for each player, and a player who had used up all of their discs could not play any more. The following rules were the same as today: when a player cannot play, the opponent plays again; all outflanked discs are flipped (a note says that flipping used to be optional); and the player with more discs wins, with equal numbers being a draw. The 1886 article said that the first 4 discs could be placed either diagonally or side by side, and that the game was a draw when neither player could play.^[1][2][5]

In Japan, the game was introduced in the Meiji era (1868–1912) under the name “Rebarushi” (a Japanese rendering of the English name, glossed as “turning over”). The 1907 book “世界遊戯法大全” [Complete Guide to the World’s Games] introduces it under the name “Urakaeshi” (“turning over”), with the reading “Revarushi”, as a game popular in Europe and America. The book says that it was devised by Mollett in Britain in 1870, and that at the time it was called “Annexation” and was played on a cross-shaped board. It describes the game as using 64 red and black cardboard pieces, starting with 2 of each placed diagonally in the center, and counting the pieces when the board is full, with the player who has more winning. It also gives the advice that the 4 corners are the safest squares. The Japan Reversi Association says that the rules in this book are the same as today’s reversi. Later, the game was also sold in Japan under the name Genpei-go (源平碁).^[4][5][6]

In Japan, the game went on sale in 1973 under the product name Othello and became enormously popular. Othello is a registered trademark, so this history uses the game’s general name, reversi. Goro Hasegawa wrote that he came up with the original form of the game in September 1945 in Mito, Ibaraki Prefecture, and MegaHouse, the current seller, gives the same account. The name was chosen by his father, Shiro Hasegawa, a scholar of English literature, after Shakespeare’s play “Othello”.^[6][7][8]

However, the utility model that Hasegawa filed in 1971 was named Genpei-go. In an article he wrote for the magazine Gendai in 1973, he said that he had planned to take Genpei-go, which his older brother played in elementary school, as the model and improve on it. Nihon Daihyakka Zensho (Nipponica) says that Hasegawa devised the game around 1972, taking a hint from a similar game that had long existed in Britain.^[6][9]

According to Hasegawa, the Japan Othello Association was founded in January 1973, and the first All Japan Championship was held on April 7. In October 1977, the first World Championship was held at the Imperial Hotel in Tokyo, and Hiroshi Inoue of Japan won.^[7][10]

Under today’s tournament rules, Black moves first from the position with 4 discs placed diagonally in the center, and a player cannot pass when a move is possible. A player who runs out of discs is given discs by the opponent and keeps playing. When a game ends with empty squares left, the empty squares are counted for the winner. The Japan Othello Association has counted this way since June 1, 2014. Before that, the empty squares were split equally between the 2 players.^{[10][11][12][13][14]}

In August 1997, at the NEC Research Institute in Princeton in the United States, Michael Buro’s program Logistello played 6 games against world champion Takeshi Murakami and won all 6. In October 2023, Hiroki Takizawa published a paper stating that he had shown by computation that the game is a draw when both players play perfectly.^[15][16]

Sources

1. The Saturday Review, “Reverses” (issue of August 21, 1886, Internet Archive) (opens in a new tab)
2. Berkeley, “Reversi and Go Bang” (Frederick A. Stokes, 1890; authorized by Lewis Waterman; Internet Archive) (opens in a new tab)
3. Michael Keller, “Reversi and its variants” (World Game Review) (opens in a new tab)
4. Masayasu Matsuura (ed.), “世界遊戯法大全” [Complete Guide to the World’s Games] (Hakubunkan, 1907), pp. 187–189, “裏返へし（レヴアルシー）” [Urakaeshi (Reversi)] (National Diet Library Digital Collections; in Japanese) (opens in a new tab)
5. Japan Reversi Association, “リバーシの歴史” [History of reversi] (1999, archived copy on the Internet Archive; in Japanese) (opens in a new tab)
6. Kotobank, “オセロ” [Othello] (entries from Sekai Daihyakka Jiten (revised edition), Nihon Daihyakka Zensho (Nipponica), and others; in Japanese) (opens in a new tab)
7. Goro Hasegawa, “オセロの歴史” [History of Othello] (Japan Othello Association; in Japanese) (opens in a new tab)
8. MegaHouse, “オセロ誕生の秘密” [The secret of how Othello was born] (official Othello website; in Japanese) (opens in a new tab)
9. hi_kmd, “日本におけるリバーシの受容と変化。文献記録で見つかるものを中心に。” [The reception of reversi in Japan and how it changed, focusing on what can be found in written records] (“紙ブグロ” [Kami Buguro] (blog), 2021; in Japanese) (opens in a new tab)
10. MegaHouse, “オセロってなに？” [What is Othello?] (official Othello website; in Japanese) (opens in a new tab)
11. World Othello Federation, “Official rules for the game Othello” (opens in a new tab)
12. Japan Othello Association, “日本オセロ連盟競技ルール” [Japan Othello Association competition rules] (in Japanese) (opens in a new tab)
13. Japan Othello Association, “オセロルール改訂について” [About the revision of the Othello rules] (2014; in Japanese) (opens in a new tab)
14. World Othello Federation, “World Othello Championships Rules” (September 2019 edition) (opens in a new tab)
15. Michael Buro, “The Othello Match of the Year: Takeshi Murakami vs. Logistello” (1997) (opens in a new tab)
16. Hiroki Takizawa, “Othello is Solved” (arXiv, 2023) (opens in a new tab)

Frequently asked questions

Does this game have other names?

In Japan, it’s also widely known as Othello, the name of a product released in 1973. Othello is a registered trademark of Othello Co., Ltd., so this page doesn’t use that name. The name used since 19th-century Britain is reversi, and on this page the game is called Disc Sandwich. In Japan, it was introduced in the Meiji era (1868–1912) under the name “Rebarushi” (“turning over”) and was later also sold under the name Genpei-go. Under today’s tournament rules, Black moves first from the position with 4 discs placed diagonally in the center. In reversi as described in the 1890 guide, Black and White took turns placing the first 4 discs anywhere on the 4 center squares, and a player who had used up their 32 discs couldn’t play any more. The rule that all outflanked discs are flipped is the same.

What happens when I have no possible moves?

You pass, and your opponent plays again. Passes can happen any number of times in a row, but you can’t pass when you have even one possible move. This page passes for you automatically, shows a banner such as “White passes (no possible moves)” at the bottom left of the board, and also keeps a note below the “Button controls” grid until the player who passed plays next. When neither player can play, the game ends, even if empty squares remain.

If the game ends with empty squares left, how are the discs counted?

This page compares the numbers of discs on the board as they are and adds the number of empty squares to the result (for example, Black 34, White 29, empty 1). In official tournament games, empty squares are counted for the winner, so this example becomes 35 to 29. The Japan Othello Association switched to this method on June 1, 2014. Before that, empty squares were split equally between the players (34.5 to 29.5 in this example). Wins, losses, and draws are the same with any of these methods.

Is there a winning strategy? Does the first or second player have an advantage?

A paper published by Hiroki Takizawa in 2023 states that he showed by computation that the game is a draw when both players play perfectly. If that result is correct, neither Black, who moves first, nor White, who moves second, has a way of playing that always wins no matter how the opponent plays (a winning strategy), and as long as both players play perfectly, neither the first nor the second player has an advantage. Practical ideas for winning (such as taking the corners, not playing early on the squares diagonally next to the corners, and reducing your opponent’s possible moves) are collected under “Strategy tips” on this page.

How do the computer’s levels differ?

There are 3 levels: Easy, Medium, and Hard. Easy doesn’t look ahead. It often chooses the square that flips the most discs, and sometimes chooses at random from the possible moves. It doesn’t specifically aim for the corners, so you can win more easily if you focus on taking them. Medium looks up to 4 moves ahead, and when 10 or fewer empty squares remain, it looks ahead to the end of the game and chooses the move that gives the largest difference in discs. Hard uses up to about 1.5 seconds per move to look deeper, and when 20 or fewer empty squares remain, it tries to look ahead to the end of the game within that time. The computer’s moves are calculated in the browser on your device.

Can I play on my phone? Do I need to sign up?

You can play in the browser on phones, tablets, and desktop. It’s free, and you don’t need to sign up or install an app. Even on devices where 3D display isn’t available, you can play with “Button controls” to the right of the board (below it on narrow screens).

Similar games

• Gomoku

  Black and White take turns placing 1 piece at a time

• Mancala

  The pieces are counted at the end, and whoever has more wins

• Checkers (Draughts)

  Played by 2 players on an 8×8 board