How to play Reversi

Reversi — the game also sold as Othello — is played on an eight-by-eight board with discs that are light on one side and dark on the other. You take light, the computer takes dark. The game starts with four discs already in the middle, arranged diagonally.

The move

Every move must outflank. You place a disc on an empty square so that, in at least one straight line from it — horizontal, vertical or diagonal — there is an unbroken run of your opponent's discs ending in a disc of your own. Every enemy disc in every line you outflank flips to your colour.

If a move outflanks nothing, it is not legal. This is the rule beginners find surprising: on a board full of empty squares, most of them cannot be played.

The board marks the squares you are allowed to play, so you never have to hunt for them.

Passing and the end

If you have no legal move, your turn is passed automatically and your opponent moves again. You do not choose to pass and you cannot decline to move when a move exists.

The game ends when neither player has a legal move — usually because the board is full, but sometimes earlier. Whoever has more discs of their colour showing wins.

Why the disc count is a trap

The single biggest mistake in Reversi is playing for discs. Flipping a huge row feels like progress and is frequently the losing move.

The reason is mobility. Every disc you flip is a square your opponent can now potentially outflank, and a player with lots of discs in the middle of the game typically has lots of exposed edges and few safe moves. The player with fewer discs often has more options, and options are what win the endgame — because the last few moves decide enormous numbers of flips.

A good rule for the first two thirds of the game is to take the fewest discs that does the job. Keeping your discs clustered in the centre and letting your opponent hold the outside is a standard winning shape.

Corners, and the squares beside them

A disc in a corner can never be flipped. There is no line through a corner that has a square on the far side, so once it is yours it is yours for the rest of the game — and it anchors every line running out of it.

Corners are therefore worth more than any number of ordinary discs, and the whole middlegame is really a fight over who is forced to give one away.

The squares diagonally adjacent to an empty corner are the dangerous ones. Playing there hands your opponent the corner in most positions, and they are the classic way a winning game is thrown away. The squares directly beside a corner along an edge are risky for the same reason, though less reliably so.

The three strengths

The computer plays with positional judgement rather than counting discs — it values corners and mobility, which is why it can look like it is losing badly and then take the last ten moves. Easy is gentle. Medium punishes a careless edge move. Hard guards its corners and squeezes your legal moves until you have nothing safe left to play.