Corners, X-squares and C-squares

Why corners matter

A disc flips only when it is trapped between two opposing discs in a straight line. A corner is at the end of every line that passes through it, so nothing can ever trap it. Once you own a corner, the discs next to it along the edges can become permanent too – see stable discs below.

X-squares and C-squares

The squares around a corner have their own names:

• X-squares are diagonally next to a corner: b2, g2, b7 and g7.
• C-squares are the edge squares right next to a corner: b1, a2, g1, h2, a7, b8, g8 and h7.

While the corner next to them is empty, these squares are risky. A disc there often gives your opponent a way into the corner. Edges in general are not bad – the danger comes from the squares right next to an empty corner.

How an X-square gives away a corner

It can happen as early as move 5. After f5 f6 e6 d6, Black plays g7, an X-square. White answers h8: the white disc in the corner traps g7, f6 and e5 along the diagonal, and White owns the corner for the rest of the game. This sequence is known as the X-square opening – a trap to avoid.

C-squares

C-squares are less dangerous than X-squares, but still risky. A disc on a C-square next to an empty corner can later be trapped along the edge, and the move that traps it lands in the corner. Before you play a C-square, check whether your opponent could then reach the corner.

When an X-square or C-square is fine

Once the corner next to it is taken, the square no longer puts that corner at risk. Late in the game, an X-square or C-square can also be your only move or simply the best one. The question is always the same: can your opponent reach the corner after your move?

Stable discs

A stable disc can never be flipped again. Corners are always stable. So is an unbroken row of your discs that runs from a corner you own along an edge: a disc on an edge can only be trapped along that edge, because every other line through it ends at the edge, and on the corner side of the row there is no room for an opposing disc. Stable discs are points you keep until the end.

How to win corners

You can only play in a corner when your disc there traps something – usually a disc your opponent has put on an X-square or C-square. So you win corners by making your opponent play next to them. Take away their other options until the only moves left are the bad ones: that is what mobility is about.

Keep learning

• Next guide: Mobility – limit your opponent's moves
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