Learn Reversi

The board starts with four discs in the middle: two black, two white, placed diagonally. Black moves first. To play, you place a disc on an empty square so that it traps one or more of your opponent's discs in a straight line — horizontal, vertical, or diagonal — between the disc you just played and another disc of your own already on the board. Every opponent disc caught in that line flips to your colour. If a placement traps nothing, it is not a legal move, and you may not play there.

• A single move can trap lines in several directions at once — all of them flip.
• The line must be unbroken: one empty square anywhere in it and nothing flips.

If you have no legal move, you pass and your opponent plays again. This is not optional — you cannot choose to pass when a legal move exists. The game ends when neither player can move, which usually means the board is full, but not always: a game can finish with empty squares nobody is able to use. Whoever has more discs at that point wins, and an equal count is a draw.

• Passing is forced, never chosen.
• Games can and do end with the board unfilled.

The single most common beginner mistake is playing whichever move flips the most discs. It feels like progress and is usually bad. Discs flip constantly, so a lead in the middlegame means very little — only the count at the end matters. Worse, grabbing a large number of discs early tends to leave you with many discs on the outside of the position and few safe moves, which hands your opponent both mobility and the edges. Strong players often deliberately keep their disc count low until the final phase.

• Count your options, not your discs.
• A position where you have many legal moves and your opponent has few is usually winning.

A disc in a corner can never be flipped: there is no square beyond it to close a line. Corners are therefore permanent, and games are frequently decided by who takes them. The corollary is what makes reversi subtle. The three squares diagonally and orthogonally adjacent to a corner — often called the X-square and C-squares — are dangerous precisely because occupying one usually gives your opponent the move that takes the corner behind it. Avoiding these squares is one of the earliest genuine skills a player develops.

• The X-square is the one diagonally adjacent to a corner. Treat it as poisoned unless you have calculated otherwise.
• Edges are more stable than the middle, but less stable than corners.

Mobility is the count of legal moves available to you. The strategic aim through the middlegame is to keep your own mobility high while reducing your opponent's, so that eventually they are forced to play a move they would rather avoid — typically one that gives up a corner. This is why quiet moves that flip only one or two discs, played toward the centre, are often strongest: they keep your position compact and your options open. The technique of forcing an opponent into a bad move by removing their good ones is the heart of skilled play.

• Flipping fewer discs often preserves more options.
• The Cognivo AI weights corners heavily and subtracts for the replies it gives you — the same two ideas.