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.

Solution
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 3 days, 19 hours ago:

Solution added. This was a hard one!


Events

All times are in Helsinki time (eet with summer time).


Pixel Go Tournament on October 31, 12:00–15:00! Tournament details and registration are here.


New pairing for the online league made every 1st and 3rd Saturday of the month.


Public lectures on Twitch every 2nd and 4th Saturday of the month at 1 pm.


Jeff and Mikko stream on Twitch on Fridays at 6 pm.