Reversi Strategy Guide

Learn the squares before the discs

The squares on a Reversi board aren’t equal. The four corners are the best squares, because a disc there can never be flipped. The squares touching a corner are the most dangerous, because a disc there is often what lets your opponent into that corner.

Players name those squares. An X-square sits diagonally next to a corner. A C-square sits on the edge right beside one. The sixteen squares in the middle, the sweet sixteen, are where the first dozen moves are played, and staying inside them early keeps you away from trouble.

The diagrams in this guide name squares by column and row. Columns run a to h from left to right, and rows run 1 to 8 from the top, so a1 is the top-left corner and h8 is the bottom-right.

Corners: the discs nobody can flip

To flip a disc, your opponent needs one of their own on each side of it along a line. A corner has no square on the far side in any direction, so nothing can ever close around it. Once you’re in a corner, that disc is yours until the final count.

A corner also makes its neighbors safe. A disc on the edge next to your corner can only be flipped along that edge, and your corner blocks one end of the line. So a row of your discs running out from your corner is as permanent as the corner itself.

That’s why one corner move can be worth more than any flip in the middle. In the position here, Black takes a1 and turns three white edge discs. Black then owns five squares in a row that can’t change color again.

• Take a corner when it’s offered, unless a different move wins the game outright.
• Before each move, check whether it gives your opponent a way into a corner.
• After you take a corner, build along its edges. Those discs are safe too.

X-squares and C-squares: the doors to the corner

Your opponent can only play in a corner by trapping one of your discs next to it. If none of your discs touch an empty corner, that corner is closed to them. Put a disc on the X-square and you’ve opened the door yourself: all they need is one of their own discs farther down the same diagonal.

In the position here, White has played b2. Black already has a disc on c3, so a1 is available at once, and White can’t undo it. The white disc on b2 flips, and Black starts building along both edges from the corner.

C-squares are a smaller risk. A disc on b1 can only be attacked along the top edge, so it’s safe while the rest of that edge is empty. It becomes a problem once your opponent has a disc farther along the edge, or can get one there.

• Treat an X-square next to an empty corner as the last move you’d choose, not the first.
• An X-square is fine once you own the corner beside it, or when the diagonal is full and can’t be used against you.
• If your opponent plays an X-square, work out how to get a disc onto that diagonal.

Edges and wedges

Edge discs are harder to flip than discs in the middle, because they can only be trapped along the edge. That makes edges worth having, but they aren’t automatically safe. An edge with a gap in it is a target.

A wedge is a disc played into a gap between two groups of your opponent’s edge discs. The discs on either side of it are the opponent’s, so they can’t close around it. From there it attacks in both directions along the edge.

In the position here, Black holds b1, c1, e1 and f1, with a hole on d1. White drops a disc into the hole. Now White threatens a1, which would flip b1 and c1, and also g1, which would flip e1 and f1. Black can’t stop both, and nothing Black plays next removes either threat.

• Before taking an edge square, look for the gaps it leaves and ask whether your opponent can play into them.
• A single empty square between your edge discs is the most dangerous kind of gap.
• When your opponent leaves a one-square hole on an edge, check whether you have a legal move into it.

Mobility: fewer discs, more moves

Mobility is the number of legal moves you have. It’s the idea that separates strong players from everyone else. If you have many moves to choose from and your opponent has few, they’ll eventually have to play a square they don’t want, such as an X-square, and hand you a corner.

You can only play where you trap opposing discs. So every disc you own on the outside of the group is a place your opponent might play from, and every disc they own is a chance for you. Having lots of discs in the middle of the game usually means giving your opponent lots of moves.

The position here is an extreme case. White has twelve discs and Black has four, and White looks far ahead. But Black can play on twenty different squares, and White has no legal move at all. White’s wall gives Black something to trap in every direction. Black’s four discs are buried where White can’t reach them.

• In the first half of the game, compare moves by how many replies they leave your opponent, not by how many discs they flip.
• When you’re choosing between two moves, prefer the one that flips fewer discs.
• Don’t build a wall. A long line of your discs facing empty squares is a list of moves for the other side.

Frontier discs and quiet moves

A frontier disc is one that touches an empty square. Your frontier discs are what your opponent plays against, so the fewer you have, the fewer moves they get. A quiet move is one that adds little or nothing to your frontier. The best quiet moves flip a single disc that is already surrounded.

Here Black has two candidate moves. Square A flips four discs in a row along the fourth row. It looks like the bigger move, but all four face empty squares, and White can then choose from twelve replies. Square B flips only the disc on f5, which is buried in the middle of the group. White is left with seven replies.

The loud move gained three more discs and gave away five more moves. In the middle of the game that’s a bad trade almost every time.

• Look for moves that flip one disc, or discs on the inside of the group.
• Avoid flipping discs that face an open area of the board.
• A move that opens a new region for both players usually helps whoever moves next, and that’s your opponent.

Stable discs: points you can bank

A stable disc is one that can’t be flipped for the rest of the game. Corners are stable from the moment they’re played. Other discs become stable when every line through them is blocked: by the edge of the board, by a full row, or by another stable disc of the same color.

Stability spreads outward from corners. In the position here, Black owns a1. The discs along the top edge and down the left edge are safe because the corner closes one end of each line. The disc on b2 is safe as well, because every line through it ends at one of those edge discs or at the corner. That’s eight discs White can never win back.

Stable discs are the only ones you can count on before the end. In a close game, count them. If you have more than thirty-two, you’ve already won.

• Grow stable groups from a corner, along both edges first and then inward.
• An X-square next to a corner you own is usually safe to play.
• A full edge is stable for whoever has discs on it, so the last move on an edge matters.

Parity: get the last move in every region

Late in the game the empty squares split into separate regions. The last move in a region can’t be answered there, so the discs it flips tend to stay flipped. Parity is the habit of arranging to be the player who moves last in each region.

The rule of thumb is short. Play into regions with an odd number of empty squares, and leave the even ones alone. If you move first in a region of one, you also move last in it. If you move first in a region of two, your opponent gets the last word.

In the position here, three squares are left: h4 on its own, and h1 and h2 together. Black is behind, 23 discs to 38. The tempting move is h2, which flips eight discs and puts Black ahead 32–30. But then White plays h4, Black has no legal move, and White takes h1 as well to win 39–25. If Black starts with the single square h4, White has to play first in the pair, at h1. Black finishes on h2 and wins 33–31.

• With a few squares left, group them into regions and count each one.
• Move first in the odd regions. Make your opponent move first in the even ones.
• Watch for passes. A pass hands the last move to the other player and changes who wants which region.

Opening principles

Black’s four possible first moves are mirror images of each other, so the game really begins with White’s reply. After Black plays f5, White has three choices. The diagonal reply at f6 and the perpendicular reply at d6 are both sound. The parallel reply at f4 is generally thought to be the weakest, because it leaves Black the better choice of moves.

Tournament players learn named lines that run ten moves deep or more, with names like Tiger and Rose. You don’t need them to play well. The same few habits carry you through the first dozen moves of any game.

• Stay inside the middle sixteen squares for as long as you have reasonable moves there.
• Flip few discs, and prefer to flip ones in the center of the group.
• Keep your discs together in one clump with your opponent’s discs on the outside.
• Don’t be the first to play an edge unless the move is quiet and leaves no gap.
• Count your opponent’s replies after each move you’re considering. Pick the move that leaves the fewest.

Endgame: now you count

The disc count means little for most of the game and everything at the end. Once about a dozen squares are left, stop judging positions by feel and start counting what each line of play leaves you with.

Counting one move is simple. If a move flips five discs, you gain six, the five flipped and the one you placed, and your opponent loses five. That’s a swing of eleven. Do the same for their best reply, and keep going to the end of the line. Include passes, because a pass means the same player moves twice.

In the position here, two squares are left and Black trails 25–37. Playing g1 flips five discs. White answers on g8, and Black loses 30–34. Playing g8 first flips only one disc, but it leaves White with no legal move. White passes, Black plays g1 anyway, and Black wins 33–31. The bigger flip was available in both lines. Only the order changed.

• Count the final score of a line, not the discs flipped by its first move.
• Look for moves that leave your opponent without a legal reply.
• When two lines finish level, choose the one that keeps the last move.