The Stoner Trap

The Stoner trap is a technique that gains a corner by playing the X-square next to the empty corner on the far side from the opponent's edge and then playing next to the end of the opponent's edge. The typical shape is when the opponent's discs run along the edge from the C-square next to one corner, and two or more squares toward the other corner are empty. For example, White has discs from D1 to G1, and A1 to C1 are empty.

First, play the X-square B2 next to the empty corner A1; at this point, the opponent must not be able to take A1 at once. Then play right next to the end of the opponent's edge on C1, aiming at the corner H1 beyond the opponent's edge. The opponent can take A1 and flip the X-square disc back, but then H1 opens, and you take that corner along with the edge. Playing the X-square G2 on the opponent's edge side is not a Stoner trap.

But the Stoner trap can fail too. The examples below show ① and ②.

• ① You cannot play next to the edge (most common): after you play the X-square, if the opponent cuts both the line to the square you would play and the X-square's diagonal with one move, a path to the nearby corner opens for the opponent. You give away that corner without ever starting the attack.
• ② The X-square is not flipped: when the opponent plays the C-square next to the nearby corner and flips your attached disc back, the X-square is not flipped if there is no opponent disc beyond it on the same line. Then the opponent does not give up the corner.
• ③ A loss even if it succeeds: the corner you gain can turn your edge into an unbalanced edge the opponent can wedge into. Often a quiet move that reduces the opponent's places to play is better than the X-square.