Square nine in the corner
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
When there are no outside liberties, White can turn this into sente seki [1]. The vital point (obviously) is a.
Note that in case of tenuki, Black cannot resist at (filling the ko)
Now Black's eye space is almost filled with a bulky five, and Black dies.
This becomes seki too, but if Black omits 6, the ko started at is slightly more advantageous for Black in that he can capture first (try to get first capture in a ko).
This variation also becomes seki (even an 1 zi better to White one, in chinese scoring), but White ends in gote.
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 . Otherwise white can kill. See 3 (iia) below.
2. One outside liberty
With one outside liberty the same main line applies, except that White must play in gote at to make it seki.
3. Two or more outside liberties
With two or more outside liberties, the attempt to make seki fails. to
are better held in reserve as ko threats.
[1]
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.
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.
When White is komaster, things are trickier. White threatens to
. 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
. 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 11 elsewhere.
After White starts with , 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?