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The first instinct of every new Reversi player is the same: flip as many discs as possible on every turn. The board fills with your colour, you feel like you are winning, and then in the last ten moves your opponent takes a corner, runs along two edges, and flips the entire board. You finish with twelve discs to their fifty-two and have no idea what happened.

What happened is that you were counting the wrong thing. In Reversi, the number of discs you have during the game is almost irrelevant. What matters is stability — how many of your discs can never be flipped back — and that concept starts and ends with corners.

Why corners cannot be flipped

A disc in the corner of the board has no disc beyond it in any direction. Since flipping requires your disc to be between two of the opponent's discs, a corner disc can never be outflanked. Once you place a piece in a corner, it is yours for the rest of the game. No amount of play by your opponent can change it.

This makes corners the single most valuable squares on the board. A player who holds two or three corners almost always wins, regardless of what the rest of the board looks like midgame.

Edges anchor off corners

Once you own a corner, every disc you place along the adjacent edge becomes stable too — as long as the line connecting it to the corner is unbroken. This is why experienced players do not just take corners, they fill edges from the corner outward. A full edge of stable discs is worth thirty interior discs that can still be flipped.

The pattern creates a cascade. Corner leads to edge, edge leads to a second edge from the same corner, and suddenly an entire quarter of the board is locked in your colour.

The danger squares: C-squares and X-squares

The squares directly adjacent to each corner are some of the most dangerous on the board. In Reversi strategy they are called C-squares (the ones on the edge, one square away from the corner) and X-squares (the diagonal neighbours of the corner).

Playing on an X-square when the adjacent corner is empty is one of the worst moves in the game. It gives your opponent a direct path to the corner, because they can outflank your X-square disc by placing in the corner. C-squares are slightly less dangerous but still risky for the same reason.

As a rule of thumb: do not play adjacent to a corner unless you are certain the corner cannot be taken, or unless you are forced to. The small handful of discs you flip is never worth the corner you hand away.

Mobility: the reason to flip fewer discs

If corners are the most important concept in Reversi, mobility is the second. Mobility means the number of legal moves you have. A player with more mobility has more choices and can steer the game toward positions that favour them; a player with low mobility is forced into moves they do not want to make — including, eventually, playing next to a corner.

Here is where the beginner instinct goes wrong. Flipping lots of discs gives you a large presence on the board, but it also gives your opponent many places to play, because every disc on the board is a potential anchor for a flanking move. Meanwhile you, with your massive presence, have fewer frontier discs to play off of. You end up with more discs but fewer moves, and your opponent gets to choose where the game goes.

Strong Reversi play inverts the intuition: keep your disc count low in the early and middle game. Flip as few discs as possible, ideally placing pieces that are completely surrounded by the opponent's discs (these are called quiet moves). This keeps your opponent's frontier large and your options open. Then, in the endgame, when you finally take corners and edges, the mass flipping happens all at once and in your favour.

The four stages of a Reversi game

Opening (moves 1–20): Both players jockey for mobility. You want to keep your disc count low and avoid edges and corners. Play toward the centre.

Early midgame (moves 21–35): Edge play begins. The question is who gets pushed into taking an X-square first. Force your opponent into bad positions by limiting their moves.

Late midgame (moves 36–50): Corners start falling. If you have maintained mobility, you can often take a corner and begin anchoring an edge. Your opponent may be forced to give up a second corner trying to find legal moves.

Endgame (moves 51–64): The board fills up. Stable discs are now all that matters. A player who owns three corners will win even if the board looked even at move 50.

Try it yourself

Play a game of Reversi on Medium difficulty with this one rule: never play adjacent to a corner unless it is your only legal move. You will be surprised how much that single constraint improves your results. Once the rule becomes habit, you can start thinking about mobility and quiet moves — and at that point, you are playing real Reversi.