EndgameTesuji2/Why is the marked stone necessary

Sub-page of EndgameTesuji2

Korlon: Can somebody show me why the marked stone in the first diagram is needed for this tesuji? I can't seem to read out why it is needed.

Bill: Without the marked stone, Black's jump is no better than the block.

[Diagram]

Block

Bill: With the block at B1 Black can secure the corner.

[Diagram]

Jump

Bill: If Black omits B5, White can play the clamp at a. After B5 Black makes the same amount of territory as with the block, but White has ko threats, starting with the cut at b.

[Diagram]

Block inferior

Bill: When Black has black+circle, the block is inferior to the jump.

[Diagram]

Jump superior

Bill: When Black has black+circle, W4 is not sente. In fact, it is reverse sente.



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