How to play Reversi well

Flipping a lot usually loses

At first the square that flips the most looks best, but discs you flip get flipped back. The more you grab in the middle game, the wider your discs spread across the board, and those discs hand the opponent plenty of places to trap them. We measured it in our own simulation. We matched a side that only counts flips against a side that looks at position, and under all six rules the side that only counted flips lost. It did not win a single game.

Corners are never flipped

To flip a disc you must trap it between two of your own, and a corner sits at the edge of the board, so there is no square beyond it. That is why a corner, once placed, stays until the game ends. It is the most valuable square in this game. Put the other way, the squares right next to a corner are dangerous, because if you play there the opponent uses that disc as a step to take the corner. If you have somewhere else to go, it is better to put that square off.

Cutting down the opponent's moves is strong

If you have no place to play, your turn is simply skipped. So if you flip only a little and keep the board narrow, the opponent has fewer squares to choose from and is forced onto a bad one. It is exactly the opposite direction from flipping a lot.

How to choose the day's rule

Three of the 6 candidate rules are drawn for the day, and you ban one and pick one. The rules differ quite a lot from each other.

• Corners first If you can play on a corner, you must play there.
• No diagonals You cannot flip diagonally. Only rows and columns count.
• Center ring closed You cannot play on the squares surrounding the four squares at the center of the board.
• Most flips only You can play only on the square that flips the most discs.
• Edges first If you can play on an edge, you must start there.
• Even squares You can play only on a square whose row and column numbers add up to an even number.

Think about the ban first. Banning the one that is worst for you is worth more than picking. When you pick, a rule that narrows the opponent is usually better than a rule that widens your own moves.

The same day is the same board

There is no randomness. The candidates are set by the date, and the opponent always plays the same moves on the same board. So if you replay a lost game you can see exactly where it turned. Also, you never run out of places to play under any rule. A rule that blocks the board is not a rule but an accident, so if narrowing would leave nothing, we made it stop narrowing.

About the name of this game

We call this form Reversi. It came out in England in 1883 and does not belong to any company. Another widely used name is a registered trademark, so we do not use it. The rules are shared, but there are places where the name is not.