Strategy
Common Reversi mistakes
The short answer
The three mistakes that lose most games: taking the move that flips the most discs, playing b2, g2, b7 or g7 while that corner is empty, and building a long edge that stops short of a corner. All three feel like progress at the time.
Every mistake below is one that a sensible person makes for a sensible reason. That is what makes them hard to drop.
1. Taking the biggest flip
The move that flips six discs looks better than the move that flips one. In the middle of a game it is nearly always worse.
Below, Black has taken the largest available flip on every move for twenty moves against the Intermediate computer, and leads 18 to 6.
Black has 5 legal moves against White's 15, and 13 discs touching an empty square against White's 4. Every one of those is a place White may play and a disc White may flip. The game ended 0 to 64.
The fix. When two moves look similar, take the one that flips fewer discs. Why that works.
2. Playing an X-square while the corner is empty
b2, g2, b7 and g7 sit diagonally next to the corners. Playing one opens a diagonal running straight into the corner.
Below, g2 flips 1 disc, f3, which makes it look like a modest little move.
The fix. Before every move, check whether the square is one of those four and whether the corner beside it is empty. The eight squares to watch.
3. Building a long edge that stops short of a corner
Edges feel safe because they can only be attacked from five directions instead of eight. A run that stops next to an empty corner is not safe at all.
Below, Black holds 5 discs in a row on the top edge: b1, c1, d1, e1, f1. The empty squares on that edge are a1, g1, h1. Black leads 30 to 15.
On the last move of the game White plays a1 and flips 9 discs, 6 of them from that top edge: b1, c1, d1, e1, f1, g1. The final score is 22 to 42.
The fix. Build edges outward from a corner you already own. Until then, one or two isolated edge discs are fine and a five-disc run is not. Edge strategy.
4. Judging the game by the score
Both diagrams above show a player who is well ahead on discs and losing. Disc count in the middle of a game is close to meaningless, because a single corner move can move it by twenty.
The fix. Count three other things instead: your legal moves against theirs, corners owned, and how many of your discs are backed to a corner and can never flip. Stable discs.
5. Rushing to the edges early
An edge disc played in the first fifteen moves is a large frontier disc, sitting next to a wide stretch of empty board. It gives your opponent extra squares and gets you nothing that lasts.
The fix. Keep your discs clustered near the centre until the middle game, then work outwards from whatever you have made safe.
6. Opening a fresh area in the endgame
Late in the game the empty squares split into separate pockets. Being the first to play into a pocket usually means your opponent gets the last move in it, and the last move into a pocket cannot be answered.
The fix. From about fifteen squares left, map the pockets and count their sizes before choosing where to play. Parity.
7. Following rules of thumb in the last ten moves
Everything on this page is a general rule, and general rules are for the part of the game that is too big to calculate. With ten squares left it is not too big any more.
The fix. Stop and count. Work out what the board looks like when the last square is filled, not after your next move. Endgame strategy.
A checklist you can actually use
- Is a corner available? Take it.
- Is this square b2, g2, b7 or g7 with the corner empty? Find another move.
- Does this square sit next to an empty corner on an edge? Be careful.
- How many replies does this give my opponent? Fewer is better.
- How many discs does it flip? Fewer is better, until the last ten moves.
Try it against the computer
Play the computer on Intermediate and keep the checklist beside you. When you lose, undo back to the move where the game turned and play the other option instead of starting again.
Next: tips for beginners, how to win at Reversi, or the full strategy guide.
Frequently asked questions
What is the most common mistake in Reversi?
Taking the move that flips the most discs. It feels like progress and it is usually the worst move available, because it spreads your discs across the empty space and hands your opponent more places to play.
Why is playing next to a corner such a bad idea?
Because it gives your opponent the line they need to reach the corner. The four squares diagonally beside a corner are the worst; the edge squares beside a corner are dangerous when they end a run of your own discs. Full explanation.
Is building a long edge a mistake?
It is when the corner at the end of it is still empty. A long unbroken run of your discs along an edge is one corner move away from flipping in a single go. Build edges out from a corner you already own instead.
Should I ever pass up a corner?
Almost never before the last dozen squares. In the endgame there are positions where taking a corner costs you the last move in two regions and more discs than the corner brings, but you should only believe that if you have counted it exactly.
How do I stop making these mistakes?
Use undo. When a game turns against you, rewind to the move that turned it and play the other option, then compare. That is much faster than starting a new game and hoping to notice. Play the computer.