Something about ko
Ko is at the heart of the game. You will need to understand its ramifications in order to progress. This is an introduction. In discussing ko we will introduce other ideas about the game and also introduce some more Japanese terms used to describe the game.
|the Black stones (a “group”) in the top right of the picture are surrounded by the White stones below them and the White group to the left of them.
|similarly the White group to the left of the Black group is surrounded by Black stones.
The difference is that the triangled position in the reference diagram is problematic. In order to have two eyes and secure life Black has to play a stone there. If White has played elsewhere on the board then Black is free to play (rule 4: not repeating a board position) and the Black group lives.
You might also notice that if Black does play there then the White group on the left does not have two eyes and will die.
By now you might be thinking this doesn’t amount to much. However suppose it is White’s turn and Black has just played elsewhere?
Just in case it is not obvious, only one of those two groups can live and it is the player that manages to fill-in that determines which group it is.
To continue the explanation let’s suppose Black can play a move elsewhere on the board which means that it would be a bad idea for White to fill-in. White would then have to respond to this move. As White has played elsewhere Black can now capture the White stone and we are right back at the beginning of this discussion.
We have just described a ko fight. To explain its significance in a game we need to introduce the idea of a ko threat.
You might recall we said that “for now” the Black group is dead but that in some circumstances White will have to finish the job. The ko fight is elsewhere on the board, but this situation provides opportunity for Black to determine its outcome.
When White took the stone in the reference diagram Black had to play elsewhere. An ideal place would be the intersection marked with a square.
If Black plays a stone there, then Black is threatening to capture the White stone marked with an X. If White does not respond than Black can capture that stone. The Black group would spring back to life. This is a ko threat.
White has to decide whether this is sufficiently valuable to prevent White filling-in (and so ending the ko-fight) or to prevent the capture here and allow the ko fight to continue.
Note: This ko fight is also a demonstration of the idea that it is not winner takes all. To get something on one part of the board you usually have to give up something in another part of the board (just choose the most valuable).
|Both players should realise the situation has something left there which could become annoying later in the game but it not immediately important to either (we describe this as aji). There are better moves on the board.
|If Black were to try to capture the marked stone in normal play it would fail and also destroy the aji (aji-keshi) and Black might not have a ko-threat in the ko-fight described above. Strong players avoid aji-keshi wherever possible.
|Similarly White does not simply kill the Black group unless necessary. An unnecessary move does not cause one’s opponent to respond and so here Black is free to play elsewhere. White would have played a move in gote. Typically, these moves have low overall value in the game.
|When Black threatened to save the group in a ko-fight, Black played a move that forced White to respond. White could not play elsewhere as the loss would have been too much. Black played a move in sente. Typically sente moves have high value in the game. Strong players are always looking for moves in sente.
|Related to the idea of sente is another concept called tenuki. If you do not have to play a move locally then play elsewhere (“tenuki”). Playing tenuki is not necessarily sente but usually has a higher value than playing an unnecessary move locally. If you do not tenuki when you can, then you are handing your opponent a free opportunity to play a big move.