How to win at Reversi

Reversi punishes the natural way to play. The move that feels best, the one that flips a long line of discs, is usually the worst move on the board. Here is the plan that replaces it.

1. Stop taking the biggest flip

This is the whole game in one habit. Below, Black has played the greedy way for twenty moves against the Intermediate computer, always choosing the move that flipped the most discs.

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Black is three discs to one ahead and completely lost. Black has 5 legal moves; White has 15. Black has 13 discs sitting next to an empty square and White has 4. Every one of those Black discs is somewhere for White to play and something for White to flip. That game ended 0 to 64.

The fix is simple and it feels wrong for about ten games. When two moves look similar, play the one that flips fewer discs. The other five mistakes follow from this one.

2. Count your opponent's replies before you move

Mobility is the number of legal moves you have. It is the number to watch, not the disc count. Before you commit to a move, look at what it opens up for the other player. A move that flips two discs and gives them one new square to play is better than a move that flips eight and gives them five.

The plan behind that is squeezing. If you keep taking their options away, sooner or later every move they have left is a bad one and they have to hand you a corner. That is how corners are actually won. Mobility, in detail.

The mechanism underneath it is worth understanding, because it makes the advice stop feeling arbitrary. A move is only legal if it sits next to an enemy disc. So every disc of yours that touches an empty square is a place your opponent may be allowed to play. Discs that touch empty squares are called frontier discs; discs completely surrounded are interior discs, and they give your opponent nothing at all.

A big flip converts a run of your opponent's interior discs into your own frontier discs. That is why the move that gains the most discs so reliably gains the most trouble. In the diagram above, Black has 13 frontier discs and White has 4, and that single comparison explains the whole game.

3. Never touch b2, g2, b7 or g7 while that corner is empty

These four squares are the X-squares, sitting diagonally next to each corner. Playing one opens a diagonal that runs straight into the corner, and your opponent takes it on the very next move. Almost every game a beginner loses badly, this is where it went wrong.

The edge squares beside a corner, b1 and a2 next to a1 for example, are C-squares. They are safer than X-squares but they turn dangerous the moment they sit at the end of a run of your own discs. Which squares, and exactly why.

4. Take corners, and hold what they anchor

A corner can never be flipped, because no line can run through it from both sides. Everything anchored to a corner in an unbroken run along an edge becomes permanent too. Those permanent discs are called stable discs, and they are the only discs that are genuinely worth something before the last few moves.

In the quiet game used further down this page, the winner finished with 22 discs that no move could have flipped. Corner strategy and stable discs cover this properly.

5. Build edges late, not early

An edge disc can only be attacked along the edge or from inside the board, so edges feel safe. A long run of your own discs on an edge with an empty corner at the end of it is not safe at all. It is a single move away from flipping in one piece.

Wait until you own the corner, then build out from it. Until then, an edge disc is worth having only when it does not leave your opponent an easy way into the corner beside it. Edge play, wedges and unbalanced edges.

6. Squeeze, and let them break first

This is what steps one to five add up to. Below is a normal game between two copies of the same computer opponent, 49 moves in. White has 4 legal moves and Black has 5. The disc count is 26 to 27, almost level.

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Nine moves later, Black has nothing left. Black's only legal move is the X-square b2, and playing it opens the a1 diagonal.

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Black plays it because there is nothing else, White answers a1, and that one move flips 9 discs: b1, c1, d1, e1, f1, g1, b2, c3, d4. The game finishes 22 to 42.

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7. Count the endgame instead of feeling it

With about twelve squares left, everything changes. Mobility stops mattering because there are barely any moves left. Discs start mattering because there is no time for anything to flip back.

This is also where parity decides games. Parity is who is forced to move last into each pocket of empty squares. Playing the last disc into a pocket flips discs your opponent has no chance to answer, so it is worth free discs every time. Parity and endgame strategy go through the counting.

Reading a position in twenty seconds

You will not calculate a Reversi position from scratch every move, and you do not need to. Four quick checks, in this order, get you most of the way to the move a strong player would choose.

1. Corners. Is one legal for me? Is one about to become legal for them? Everything else waits until this is answered.
2. Dots. How many legal moves do I have, and roughly how many do they have? A gap of five or more is the whole story of the position.
3. Shape. Are my discs in a tight block or spread across the board? Tight is good. Spread means a wide frontier and a lot of squares handed over.
4. Candidates. Pick the two or three smallest legal moves and ask what each one opens up. Play the one that opens the least.

Notice that the disc count is not on the list. Until the last stretch of the game it is the least informative number on the board.

Common ways this goes wrong

• Taking an edge to look solid. An early edge disc is a big frontier disc and it usually invites the opponent in rather than shutting them out.
• Fixing a mobility problem with an X-square. Running short of moves is bad. Handing over a corner to buy one more move is worse.
• Panicking at a bad disc count. Being 8 to 30 down on move 30 is often a winning position. Check the dots before you change plan.
• Still playing for shape on move 52. Once about twelve squares are left, shape has stopped mattering and arithmetic has taken over. The full list of mistakes.

The plan in one table

Stage	What to aim for	What to avoid
Moves 1 to 20	Few discs, all clustered in the middle	Long flips, edges, anything beside a corner
Moves 20 to 45	More legal moves than your opponent	X-squares, unbalanced edges, sudden big flips
Moves 45 to 60	Corners, stable discs, the last move in each region	Opening a fresh region for nothing

Practise it against the computer

None of this sticks by reading it. Play the computer on Easy and win three games without ever playing an X-square. Then move to Intermediate and count your legal moves before every single move. Undo is free and unlimited, so when a game turns, rewind to the move that turned it and try the other option.

When you want the reasoning behind each piece of this, the strategy guide is the hub for the whole set. If you have only just learned the rules, tips for beginners is a shorter start, and the rules page covers flipping, passing and scoring.