Strategy
Reversi corner strategy
The short answer
A corner disc can never be flipped, because no line can pass through a corner from both sides. That makes a1, h1, a8 and h8 the only squares worth taking for their own sake. You win corners by taking your opponent's safe moves away until they have to open one for you.
Corners are the fixed points of the game. Everything else on the board can change colour a dozen times; a corner changes never.
Why a corner is permanent
To flip discs you place one so that a straight line of your opponent's discs is trapped between your new disc and one you already own. That line needs a square on each side.
A corner sits at the end of every line that touches it. There is no square beyond a1 going up or going left, so nothing can bracket a1 along the top edge, the left edge or the long diagonal. The disc placed there is fixed for the rest of the game. It is the only square on the board with that property.
Stability spreads out from a corner
A disc that can never be flipped again is called a stable disc. Corners are stable immediately. Then the effect runs along the edges: a disc on the edge with an unbroken run of your own discs back to a corner cannot be bracketed along that edge either, so it becomes permanent too.
Below, White has just taken h1 in a real game. Look at the whole right edge.
One move, 7 discs that are now beyond reach. Four moves later, after White has also taken h8 and a8, the count is 15. By the end of the game White holds 22 permanent discs and wins 22 to 42. How to count stable discs goes through this square by square.
How corners are actually won
You will very rarely walk up and take a corner because your opponent was not looking. You take it because they ran out of anything else to play.
Here is the moment in the same game. White is to move with 4 legal moves, and 2 of them are corners: h1 and h8. Black did not give these away by blundering. Black simply had nothing left that did not open something.
The plan that produces this position is mobility: keep more legal moves than your opponent, and keep taking theirs away with small quiet moves. They run out first, and then the corners fall. Mobility explained.
The squares that give corners away
The fastest way to lose a corner is to play the square diagonally next to it. Those four squares, b2, g2, b7 and g7, are the X-squares. Playing one opens the diagonal that runs into the corner.
The edge squares next to a corner, b1 and a2 beside a1, are C-squares. They are less immediately fatal but they cause the same problem when they sit at the end of a run of your discs. The full picture on both.
What a corner is worth in real numbers
At the end of the same game, White was ahead by a single disc, 32 to 31, with one square left. That square was a1. Playing it flipped 9 discs at once: b1, c1, d1, e1, f1, g1, b2, c3, d4.
A single corner swung the result by 19 discs. That is normal, not unusual, and it is why disc count before the corners are settled tells you almost nothing.
Four rules for corners
- Take a corner when it is offered, unless you have counted the whole endgame and know better.
- Never play an X-square while that corner is empty. b2, g2, b7, g7.
- Be careful with C-squares at the end of your own edge run: b1, a2, g1, h2, a7, b8, g8, h7.
- Build out from a corner you own, not towards one nobody owns. An edge that reaches a corner is permanent; one that stops short is a target. Edge strategy.
Try it against the computer
Play the computer on Beginner first. It will walk past a free corner, so you can practise spotting them. Then move to Intermediate, which never gives one away by accident, and see how long you can go without opening one yourself.
Next: X-squares and C-squares, stable discs, or back to the strategy guide.
Frequently asked questions
Why can a corner never be flipped in Reversi?
Flipping needs your opponent to trap a line of discs between two of their own. A corner has no square beyond it in any direction, so no line can ever reach it from both sides. Once a corner disc is placed it stays that colour for the whole game.
Should I always take a corner when it is offered?
Nearly always. The corner disc itself can never be flipped and it makes the discs anchored to it permanent as well. The only real exception is deep in the endgame, when taking it costs you the last move in a region and more discs than the corner is worth.
How do I get my opponent to give me a corner?
Take their choices away instead of taking discs. Play small quiet moves inside your own area so they have nothing safe to aim at. When every move they have left touches a corner, one of those corners becomes yours. More on mobility.
Is the corner worth more than the discs it flips?
Usually far more, because the corner keeps paying. Discs flipped by the corner move can be flipped back, but the corner disc and the unbroken run beside it cannot. A corner is normally worth somewhere between five and fifteen discs by the end of the game.
What if all four corners are still empty late in the game?
Then the game is about who runs out of safe moves first. Count your legal moves against your opponent's. The player with fewer choices will be forced to open a corner, and it is usually decided ten moves before it happens. Parity often settles it.