Square nine in the corner

    Keywords: Life & Death, Ko

Square nine in the corner is related to the carpenter's square. See square nine for a discussion of the abstract shape.

1. No outside liberties

Square nine  

When there are no outside liberties, White can turn this into sente seki [1]. The vital point (obviously) is a.

Main line  

The points B2 and W3 are miai. B6 avoids White starting a ten thousand year ko at a. B6 can be at b.

Oops! (Black 6 tenuki)  

Note that in case of tenuki, Black cannot resist at B8 (filling the ko)

Now Black's eye space is almost filled with a bulky five, and Black dies.

Variation at 5  

This becomes seki too, but if Black omits 6, the ko started at B6 is slightly more advantageous for Black in that he can capture first (try to get first capture in a ko).

Variation at 4  

B4 here is not so good, as it invites a two stage ko.

Variation at 3  

This variation also becomes seki (even an 1 zi better to White one, in chinese scoring), but White ends in gote.

Variation at 3 (ii)  

Black trying to avoid seki by taking away the eye is not a good idea because it becomes seki anyway and Black takes gote too. Black must play at B6. Otherwise white can kill. See 3 (iia) below.

Variation at 3 (iia)  

Black is dead after W1. White would not normally have to play out the rest of the moves.

2. One outside liberty

Main line  

With one outside liberty the same main line applies, except that White must play in gote at W7 to make it seki.

3. Two or more outside liberties

Main line  

With two or more outside liberties, the attempt to make seki fails. W1 to W7 are better held in reserve as ko threats.


This position is hyperactive, depending on who is komaster. If Black is, or if neither player has any ko threats, White can make seki in gote.

Gote seki  

Think about it. What does White threaten?

Seki variation I  

White can try W1, but B2 takes away the eye. Now what?

Seki variation II  

This leads to a 10,000 year ko, which White will be reluctant to start. Or White can play at a to preserve the seki.

White as komaster: the threat  

When White is komaster, things are trickier. White threatens W1 to W5. Now there is a 10,000 year ko that favors White. Unless Black can become komaster, he should often go ahead and bite the bullet with B6. Since he will lose the ko, the earlier he does so, the larger are the plays he gets in exchange.

By the same logic, Black should normally reply to White's invasion, setting up a ko to lose while there is still time.

Black makes ko to lose it now.  

B7, B9, Black 11 elsewhere.

After White starts with white+circle, Black gets three plays elsewhere in exchange for the corner. The right time to do this is when the miai value of these plays is around 7 points.

When he cannot get so much in return for the ko, Black should usually make seki in gote, as shown in the Main Line diagram.

See also: Pear Shape?

Square nine in the corner last edited by hnishy on June 18, 2024 - 06:44
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