Legal moves in Reversi

One rule decides every move in Reversi, and it is worth getting exactly right, because almost everything else in the game follows from it. You do not choose where to play. The board tells you where you are allowed to play, and a large part of good strategy is about shrinking that list for your opponent.

The test, in three parts

To play square X, all three of these have to be true:

1. X is empty. You can never play on an occupied square.
2. At least one straight line from X starts with an unbroken run of your opponent's discs. One disc is enough. Five is fine too. There can be no empty square and none of your discs inside that run.
3. The square immediately past the end of that run holds one of your discs. If the run reaches the edge of the board, or the next square is empty, the line does not close and it does not count.

A line can run left, right, up, down, or along either diagonal in either direction. That is eight directions to check from every square. Only one of them needs to work for the move to be legal.

The starting position

Sixty squares are empty at the start and only 4 of them are legal for Black: d3, c4, f5, e6.

abcdefgh

12345678

Take d3 and run the test. The square is empty. Looking straight down, d4 holds a white disc, so there is a run of one opposing disc. The next square down, d5, holds a black disc, which closes the run. All three parts pass, so d3 is legal and playing it flips d4.

Why the squares next to it are not legal

Look at c5. It touches the white disc on d4 on a diagonal, so it feels like it should work. Follow that diagonal past d4 and the next square, e3, is empty. The run never closes, so nothing flips and the move is illegal. The engine confirms it: c5 would flip 0 discs.

Now look at d6. Directly above it is d5, which is one of your own black discs, so there is no opposing run in that direction at all. On the diagonal up and to the right sits the white disc on e5, but past it f4 is empty. Again no closed line, and d6 flips 0 discs.

One move, two directions

A single square can close more than one line, and when it does, every one of them flips. After the moves d3 e3, Black can play f3, which closes a run along the row and a run along the diagonal at once. Both e3 and e4 turn over.

abcdefgh

12345678

There is no version of this where you take one line and leave the other. If a direction closes, it flips. Late in the game, when the board is crowded, a single move can close five or six lines and turn over more than a dozen discs at once.

A midgame board

Openings are tidy. Real positions are not. Here is a board twelve moves in. Black is behind 6 discs to 10 and has 17 legal squares to pick from, all marked.

abcdefgh

12345678

Notice d3. It is empty and it touches white discs, both on the row at e3 and on the diagonal at e4, so it looks playable. It is not. Following the row, e3 is white and f3 past it is empty. Following the diagonal, e4 and f5 are both white and g6 past them is empty. Neither run closes, so d3 flips 0 discs and is not on the list.

Also notice how many of the legal squares sit along the bottom edge. That is normal. Legal moves cluster wherever the two colours meet, which is why the shape of a position matters so much more than the disc count. Mobility is the strategy built on this.

Finding your moves quickly

On a physical board, do not scan all 64 squares. Every legal move has to touch an opposing disc, so:

1. Look only at empty squares next to your opponent's discs. That usually cuts the board down to ten or fifteen candidates.
2. From each candidate, follow the opposing discs outward. Keep going while they are your opponent's colour.
3. Check the square just past the end. One of yours, and the move is legal. Empty, or off the board, and it is not.

With a bit of practice you stop doing this consciously and start seeing the lines. The board on this site marks every legal square for you while you learn, which is the fastest way to build the habit.

When you have none

If no square passes the test, you have no legal move and you pass. Your opponent moves again and you rejoin as soon as a square opens up. Here is a real position ten moves into a game where Black has nothing at all, so White plays twice in a row.

abcdefgh

12345678

This is not a bug or a penalty. It is a normal part of the game and a large part of what makes it interesting. The passing rule covers what happens next, and when a game ends covers the case where neither player has a move.

See the legal moves live

The board on the home page marks every square you are allowed to play, on a phone as well as a laptop, and it will not let you place a disc anywhere else. Play a few moves against the computer and watch the marks move around as the position changes. That picture, more than any written rule, is what teaches you to see the lines.

Related: the complete rules, board setup and the best first move.