Simple seki without eyes  

The simplest form of a seki is shown in this diagram: the and groups have no eyes and share two liberties at a and b. If either player plays on one of these points, the opponent will play the other and capture. So neither player will.^[2]

White captures a bulky five  

If Black nevertheless plays at , White will capture the bulky five with .

Similar, if White starts at , Black will capture the farmer's hat at .

Though playing first locally fails, it can be used as a big ko threat, losing points however.

Seki with one eye each

One eye versus another  

A slightly more complex variant is formed by the marked groups. Both have one eye and they share one liberty (at a). Again, if either player plays at a, the other will capture.

Seki with and without eyes

Two eyes versus none  

Both white groups have one eye (at a and b). The black group has none, but shares a liberty with each white group. Again, since no player can capture any stones of the other player, the position is seki.

Seki with one (false) eye

One false eye versus none  

The false eye at b would count in Chinese scoring, but not in Japanese (regardless of being false or not). Stone scoring does not count it either, because White depends on it.

The only play which doesn't immediately lose the corner, is White at a. However, that costs one point in gote (RP: only under Japanese rules), as Black captures at b and White must recreate the original position by recapturing at . This is an example of sending two returning one.

Also note that under very rare circumstances, in this kind of position White can use the superko rule to capture the black stones in some rulesets. See Rules Beast 1.

White's blunder  

Playing at is a serious mistake: now White is simply dead.

White shouldn't play at c either: She would put her big group into self-atari, letting Black capture it at .

No seki  

Here b is a real eye. This is no seki since White is able to capture Black.

Seki with two (false) eyes

Two false eyes versus none  

Both a and b are (false) eyes, which would count in Chinese (and de facto Internet) rules, but not in Japanese scoring (regardless of being false or not). Stone scoring is in-between: It would let White only count one eye (as much she can afford to fill).

Note again that in some rulesets this may not be seki under rare circumstances, depending upon how its superko rule treats sending two returning one.

Seki with partly filled eye space

3 inside 5  

Another type of seki. Black can capture the three white stones, but if he does, White will come back at (nakade), leaving Black with a dead shape, so Black won't do so.

Likewise, if White were to try to capture Black, she would at some point need to play another stone inside Black's eye space. However, Black would then happily capture four stones and be left with a live straight-four eye space.

Therefore, neither side wants to play here and this position will remain until the end, with the three white stones and the black group surrounding them alive through seki.

When scored, the black group gets zero points.

Seki with double ko

Double-ko seki  

This is an interesting seki involving double ko. If White captures at a, Black will capture at b. Later, if White recaptures at , Black recapture at , returning to the original position. Similar if Black starts.

See double ko and seki with eyes question 1 for more.

Seki with strings in atari

Hanezeki  

Black has a string in atari, White too, but for either side, capturing starts a capturing race that the other side can win (see hanezeki). Best play is to leave this, therefore it's seki.

Neighbouring seki

Complex seki  

Here is an example of one seki (two eyes versus none) touching another (one eye each). These are effectively neighbouring seki, not one single seki. Being neighbour of a seki isn't being part of it, even if the neighbour happens to be part of another seki itself.

Rules aspects

Japanese seki definitions

Different Japanese rules describe seki differently. The Japanese 1989 Rules do not define seki but define stones to be in seki if they have dame (where, informally, dame are points that aren't inside eyes).

It is possible for players not to fill dame points before passing. However, a pedantic player is pefectly within his rights to demand that any group of his opponent (even if it has 15 eyes) is in seki if it is touching dame, and therefore its territory isn't to be counted. ^[4]

However, this isn't really important as there is another Japanese rule (9.3) that says, quote,