Reversi rules

This page is the whole ruleset in one place. Every diagram on it comes from the same rules engine that runs the playable board, so what you read here and what the board does are the same thing.

What you need

A board of 8 rows and 8 columns, so 64 squares, and 64 discs. Each disc is black on one face and white on the other. There is no separate set of black pieces and white pieces. There is one pile of discs, and which colour a disc counts as depends only on which way up it is sitting right now.

One player is Black and one is White. In a boxed Othello set the discs are split 32 each at the start, which is why people sometimes say each player has 32 discs. The disc count page goes into what that split does and does not mean.

Setting up

Four discs go in the middle four squares, arranged so that the same colours sit diagonally opposite each other. White on d4 and e5, black on d5 and e4. Every other square is empty.

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That is the only legal start in modern play. Older Reversi sets let each player place their first two discs anywhere inside those centre four squares, which allowed three different openings instead of one. Board setup step by step covers the layout and the two ways people get it wrong.

Black moves first

Black always moves first. There is no toss, no choice and no version of the modern game where White opens. From the starting position Black has exactly four legal moves: d3, c4, f5, e6.

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Those four squares are the same move seen from four sides. The starting position is unchanged by a half turn of the board or by a reflection in either long diagonal, and reflecting in the a1 to h8 diagonal turns d3 into c4, so none of them is better than the others. Why Black goes first explains the symmetry, and the best first move looks at what happens after each one.

What makes a move legal

A move is legal when it traps at least one of your opponent's discs. Trapping means this, exactly:

1. You place your disc on an empty square.
2. Starting from that square and running in a straight line, there is at least one unbroken run of your opponent's discs.
3. The square immediately past the far end of that run holds one of your discs.

The line can run left, right, up, down, or along either diagonal, so there are eight directions to check. A gap of one empty square breaks the line and the direction does not count. One of your own discs partway along breaks it too.

Take Black's opening move d3. The white disc on d4 sits directly below it, and the black disc on d5 sits below that, closing the line. So d3 traps d4, and d4 flips. The engine agrees: playing d3 from the start flips exactly d4.

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Now compare c5, which touches the white disc on d4 diagonally. Follow that diagonal past d4 and the next square, e3, is empty, so the line is never closed. Playing c5 from the start would flip 0 discs, which is why it is not a legal move. Legal moves in detail works through more cases like this.

Flipping

Every disc your move traps flips, in every direction at once. This is not optional. If a move traps three discs to the left and one on a diagonal, all four turn over, even in the rare case where you would rather they did not.

Here is a two-direction flip from a real game. After the moves d3 e3, Black plays f3, which closes a line along the row and a line along the diagonal at the same time. Both e3 and e4 flip.

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Three things about flipping that catch new players out:

• Only discs in the trapped runs flip. A disc that happens to be next to your new disc but is not in a closed line stays as it is.
• Flipped discs do not extend the move. You work out every line from the position as it was before you placed the disc. A disc that flips does not then trap more discs behind it.
• A disc never moves. Once placed, it stays on that square for the rest of the game. It can flip back and forth any number of times.

Passing

If you have no legal move, you pass and your opponent moves again. You do not lose your turn permanently, and you rejoin as soon as a legal square opens up.

You may only pass when you have to. If even one square would flip a disc, you have to play it. This is the rule that makes running your opponent out of moves a real strategy rather than a curiosity.

The position below comes from a real game after ten moves. Black is ahead 10 discs to 4, and has no legal square anywhere on the board, so White plays again. White's options are marked: c3 and d6.

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The passing rule in full, including how passes are written down in a game transcript.

How the game ends

The game ends when neither player has a legal move. Usually that is when the board is full after 60 moves, but not always. Two other endings are perfectly normal:

• Both players are stuck. Squares are still empty, but nothing either player could place would flip anything.
• One colour is wiped out. If a move flips the last disc of the other colour, that player has nothing left to flip from, so the game is over immediately.

A wipeout can happen fast. The game below is over after 9 moves, with 13 black discs, 0 white and 51 squares still empty.

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When a game ends covers all three endings with the counts.

Scoring

Count the discs. The player with more discs of their colour showing wins. An equal count is a draw, and because there are 64 squares a drawn full board is always 32 to 32.

Here is the finish of a complete game. Black ends with 34, White with 30, and the board is full.

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Tournaments add one wrinkle. If the game finished with squares empty, those squares are awarded to the winner, so a game that stops at 33 to 29 with two blanks is recorded as 35 to 29. That never changes who won, only the margin, which matters when a tournament is decided on disc difference. Scoring worked through, and the official tournament rules.

The rules that are easy to get wrong

Four points come up again and again, usually because a house rule from somebody's childhood has stuck.

• You cannot pass by choice. No legal move means pass. A legal move means play.
• You cannot flip only some of the discs you trapped. All of them turn over.
• You cannot jump over a gap or over your own disc. The run between your new disc and your closing disc has to be nothing but opposing discs.
• Being ahead on discs partway through means very little. One move late in the game can flip fifteen discs. The count only settles at the end.

That last one is a strategy point rather than a rule, but it is the single thing that separates a first game from a tenth. The strategy guide starts there, and common mistakes lists the rest.

Reversi, Othello and which rules you are playing

Reversi was published in England in the 1880s and is in the public domain. Othello is a tidied-up version devised by Goro Hasegawa in Japan and sold from 1973, and it is a registered trademark. The rules on this page are the Othello rules, which is what essentially everyone plays today, under whichever name their set came with.

The differences with the older game are small. Othello fixes the opening position; Reversi let players choose it. Othello gives each player 32 discs; Reversi used whatever was in the box. Under some old Reversi rulesets a player with no move lost or the game simply stopped, where Othello has them pass. The full comparison and the history are on their own pages.

Every rule on one screen

Try it on the board

Reading the rules gets you about a third of the way. Play four or five games and they stop being rules and start being obvious. The board on the home page marks every legal square for you, so you can concentrate on what a move does rather than on whether it is allowed. Undo is free and unlimited, so try the move you are unsure about and look at what flips.

Start against the computer on Beginner or Easy, or play a friend on the two player board. If you want the rules on paper, there is a printable rules sheet and a printable board.