Corners, X-squares and C-squares

A disc on a corner can never be flipped. The squares next to a corner, the X-squares and C-squares, are the usual way to hand that corner to your opponent.

Why corners are strong

A corner (a1, h1, a8, h8) cannot be outflanked in any direction, so a disc there stays to the end. Discs along the edges next to your corner become hard to flip too, and the number of stable discs (discs that can never be flipped) grows from there.

You can only play a corner when it outflanks your opponent. So your own discs around a corner are what let your opponent take it.

X-squares (b2, g2, b7, g7)

The square diagonal to a corner is an X-square. Playing it while the corner is empty often opens the diagonal for your opponent. In the opening and middlegame it is one of the costliest moves.

Try it on the boardWhite to move · 13 empty squares

White to move. b7 ends -2 (a loss by 2); the best move e1 ends +6 (a win by 6).

b7 is an X-square (diagonal to the corner a8). While the corner is empty it tends to give the corner away.
After this move the opponent can play the corner a8.

Best line: e1 a1 b2 b7 a8 b8 a5 g7

Press a square with a dot to see the result of that move.

C-squares (a2, b1, g1, h2, a7, b8, g8, h7)

The squares beside a corner along the edges are C-squares. Depending on the edge they can be fine, but with opponent discs on that edge they often let the corner be taken.

Try it on the boardBlack to move · 12 empty squares

Black to move. a7 ends -14 (a loss by 14); the best move e8 ends +14 (a win by 14).

a7 is a C-square (next to the corner a8). In this shape it lets the corner be taken.
Opponent's moves: 5 after this move, 3 after e8.

Best line: e8 b7 a8 a7 h2 f8 g2 f2

Press a square with a dot to see the result of that move.

Checking in a game

On your turn, first look for a corner you can take.
For each candidate, check whether a corner becomes playable for your opponent.
Play X- and C-squares when the corner is already taken or nothing better is left.

There are exceptions: once the corner is filled, the X-square is harmless, and in the endgame giving a corner can still win on discs. In the endgame, confirm by reading.

Endgame puzzles on this theme

No. 1● Black · 10 empty
No. 2○ White · 11 empty
No. 3● Black · 12 empty
No. 4● Black · 12 empty
No. 6○ White · 13 empty
No. 7● Black · 14 empty
No. 8● Black · 14 empty
No. 10○ White · 15 empty
No. 11● Black · 10 empty
No. 12○ White · 11 empty
No. 13● Black · 12 empty
No. 14○ White · 13 empty
No. 17○ White · 11 empty
No. 18● Black · 12 empty
No. 19○ White · 13 empty
No. 41● Black · 10 empty
No. 43● Black · 12 empty
No. 44○ White · 13 empty
No. 45● Black · 14 empty
No. 46● Black · 10 empty
No. 47○ White · 11 empty
No. 48● Black · 12 empty
No. 49○ White · 13 empty
No. 50● Black · 14 empty

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