FlipSight : The Danger of X- and C-squares

X-square: the corner opens along the diagonal

If you play on the X-square diagonally next to an empty corner, the opponent can often reach the corner along that diagonal. It is the most common way to give away a corner.

Black’s 25th move. If Black plays the X-square B2, White takes the A1 corner, flipping B2 and C3 on the diagonal, and gains a big advantage. Black G6 instead opens no corner for White and keeps Black’s advantage. Use ▶ to follow Black B2, White A1.

Exception: giving up a corner to set up a wedge

The next exercise is a position from a different game. After Black played G2, White took the H1 corner. But if Black wedges in at G1, the white discs on B1 to F1 sit between Black G1 and the empty A1, so Black can aim at the A1 corner. When you give up a corner like this, compare what you gain in return, together with the whole position.

Exercise

After Black G2, White took the H1 corner. Black to move. Where on the top edge should Black wedge in to make a foothold aiming at the A1 corner?

C-square: dangerous when the squares next to it are empty

A C-square can also lead to losing a corner. A C-square whose neighboring corner and A-square are empty, and which is cut off from the other discs on the edge, is called a lone C-square; be especially careful with it. The opponent can play next to it on the A-square and make a foothold that traps the C-square disc. With your own discs connected along the edge you can sometimes defend, but you have to check the opponent's actual legal moves and replies.

Move 30, White to move. Black H7 is a lone C-square with H6 and H8 empty. If White plays next to it at H6, Black's best defense H5 flips H6, but the white disc on H4 remains, so White can trap H5, H6 and H7 from H8. Even if Black replies differently, White can flip H7 with H8, and H8 opens no other corner for Black. Use ▶ to follow White H6 → Black H5 → White H8.