Now, I will describe the additional rule called Ko - which means eternity in Japanese.
That’s the last rule to remember. If you read this page, you will have understood all the rules of Go.
The concept of Ko is a little more difficult than the previously described rules.
A white stone is in Atari. Capture it
Have you captured it? Now look carefully at the board.
You'll see that the black stone F6 just landed on Atari!
It might look like white could capture it, but...
What happens if the will white play on E6?
It will result in the same position on the board as it started.
In other words, both black and white would capture each other forever!
Therefore, the moves that produce the same position on the board are prohibited.
This is the concept of Ko (eternity).
On the board above, you can also make moves for white. Then, try to get the black stone in the middle.
You will see the message - Illegal move - KO.
Will white never be able to capture that black stone again?
They will. But they need to play somewhere else first.
Thus, this will cause a different board position. Therefore, you can recapture the Ko stone once you play elsewhere first.
Make a move wherever you want for White on the board below.
And also make a move with Black as well. Now it’s white’s turn. They can now get the black stone back.
Confirm it yourself on the board above.
Now it’s Black’s turn. However, they cannot capture the white stone immediately.
Confirm yourself on the board. You will see the message again - Illegal play - KO.
They can capture the white stone if they make moves elsewhere like the white ones did.
On the board above, you can make moves for both white and black.
Try to understand the concept of Ko’s rule by playing it.
You can undo your moves by pressing the Back button.
How does Ko appear in a real game?
Look at the diagram. Five black stones at the top of the board are about to die.
First, capture a white stone playing on E8.
Now the Ko battle has begun!
Five white stones are in danger and they can't capture the black stone playing on D8 because we're in a case of Ko.
So they play somewhere where black can't ignore like E2.
How 8 black stones will be captured if this is ignored, Black plays on E1.
Now, White can capture the Ko stone back by playing on D8.
So, Black will play in a place where White cannot ignore, like for example B4. If White responds A4, Black can capture the Ko stones again by playing on E8
We call this a Ko battle.
The most important thing is: to win a Ko battle, find a threat - a place where you can win big with 2 successive moves. If the opponent doesn’t allow it, Ko’s battle continues. If he ignores the threat, you can make two successive moves while he gains the Ko.
Ko is difficult as a Ko battle involves a lot of strategic moves across the board.
For starters, just remember that moves that produce the same position on the board are prohibited. That’s it. The Ko rule does not allow situations like this to happen.
Ko is not just a restrictive rule of Go, but an extra emotion in a game.