Sunday Problem #19

Published 11 Oct 2020 by antti (last edited 19 Oct 2020)

The carpenter’s square or ichigō-masu (, meaning a small wooden container with a volume of 0.1804 litres) is one of the more dreaded tsumego shapes, especially by kyū players, which makes it an excellent topic for a Sunday Problem.

Should Black go for the carpenter’s square, or is there a better option available?

Black to play.

Standard sequence
Black’s block of 1, which creates the carpenter’s square, is the most straightforward choice. White’s eye-robbing placement of 2 is the most promising attack, and after this the reading becomes complicated.
In general, in most carpenter’s square variants, the black 1-2 tesuji attachment of 3 is then the best move, and some kind of a kō fight results. 4–8 here are thought to generally lead to the best kō fight for both players.
Black dies
Note that black 5 in the previous sequence is forced. If Black tries to block with 5 here, then white 6–10 kill Black unconditionally.
Bad style
In this particular carpenter’s square shape, even the less stylish bump of black 3 manages to create a kō, as shown here with up to white 10. However, it is generally advisable to forget about black 3 as a move.
Black 3 dies with no outer liberty
The reason to this is that, in any case, Black can at most create a kō for life; but if the carpenter’s square is of the basic variety where Black has no liberties, the sequence does not even work.
With white
a in place, White can create a capturing race with 6 as shown here, and, up to 10, Black’s group is unconditionally dead.
Also kō
Black can also consider the diagonal of 1. This time White attaches at 2 and, up to 8, we have another kō fight.
Technically correct
Strictly speaking, Black’s best move is to claim the key point of 2-2 for herself. White’s counter to this is flashy, attaching with 2 and 4; in response to this, the best Black can do is fight with 3 and 5, and then white 6 sets up a kō.
Technically this is the best kō for Black that we have seen in all the variations up to this point, because Black gets to capture the kō first; this makes a difference of one threat, which can easily become a crucial factor in who finally wins the kō.

Comments (1)

antti wrote 6 months, 3 weeks ago:

Solution added. This was a hard one!