Ladder escape fork
A forking move which creates a ladder escape.
Contents
Example
In the following position, Red has no edge template.
The only option seems to be a ladder.
However, pushing the ladder too much is useless, and it actually enables Blue to win.
Red needs the two pieces at the top right hand-corner of the board. Red pushes the ladder just enough to use a ladder escape fork. Piece number 3 is called the pivot piece. It threatens to connect to the top group and acts as a ladder escape as well.
How far can the attacker climb?
The effectiveness of a potential fork can be measured by how far the attacker can potentially climb. Let us consider some typical ladders and potential forks:
2nd row ladder
In the first example, Red's space is limited. Red can climb to the 4th row, potentially bridging to a stone on the 6th row.
If Red has slightly more space, Red can climb to the 5th row, potentially bridging to a stone on the 7th row.
Finally, in a switchback situation, where a 2nd-to-4th row switchback would allow Red to connect, Red can climb to the 6th row, potentially bridging to a stone on the 8th row. This kind of forking ladder escape is called a switchback fork.
Of course, there are many variations of this, depending on what other pieces Red and Blue have on the board. But the three basic patterns shown above are the most common, and are good starting points for planning more complex ladder escape forks.
3nd row ladder
The situation for 3rd row ladders is largely similar to that of 2nd row ladders. In the most constrained case, Red can climb to the 5th row, potentially bridging to a stone on the 7th row.
If Red has slightly more space, Red can climb to the 6th row, potentially bridging to a stone on the 8th row.
Finally, in a switchback situation, where a 3nd-to-5th row switchback would allow Red to connect, Red can play a switchback fork and climb to the 7th row, potentially bridging to a stone on the 9th row. This is extremely threatening; note that on an 11×11 board, the 9th row is almost on the opposite side of he board.
4th row and higher ladders
The situation for 4th row and higher ladders is essentially similar, provided that the attacker can guarantee that the pivot piece connects to the edge. For example, in the following situation, the pivot piece "1" is not connected to the edge, and Blue could foil by playing at "a":
However, if Red had, for example, one more piece at either of the locations marked "*" (or pretty much anywhere else near the bottom edge), then the pivot piece would be sufficiently connected for the fork to work in the same way as for 2nd or 3rd row ladders.
