On the following board, assume it is white’s move and notice that the two white stones are in atari. Where can white play to save their stones?
Answer.
Playing at D3 gives white’s stones three liberties and a chance to escape.
Section 2.2 Capturing Stones
A group of stones is captured when all of its liberties are taken by their opponent’s stones. When a group has only one liberty remaining, the group is one move away from being captured and we say the group is in atari. We also use the word atari as a verb in statements like, "I will atari your stones with the following move."
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(If you are as old as me, you might be familiar with Atari as the name of a video game company. Atari was a term in Go first!)
When stones are in atari we have to decide whether we want to try to save them from capture. In order to save a group in atari we must play to give the group more liberties. We can do that by capturing some of the stones taking our liberties, or (possibly) by extending our group at the point of its last liberty.
Example 2.2.1. Saving Stones in Atari.
Example 2.2.2. Stones in Atari.
Consider the following situation. Which group of stones is in atari?
The black group on the left is the only group in atari. (The white group on the bottom has two liberties.) Notice that there is nothing black can do to gain more liberties! Black can’t place a stone at A3 since that would be a self-capture, and black cannot capture stones in one move.
Suppose black plays at A1. Now which stones are in atari?
Now the four white stones at the bottom and the three black stones are both in atari, but it’s white’s turn. Can white play at A3? Yes! White A3 captures the three black stones!
Intersections on the edge of the board have fewer adjecent intersections, so stones there have fewer liberties. Here is a short video showing some common capturing races near the edge of the board, and you can see how the edge of the board is used to help capture stones.
Subsection 2.2.1 Ladders
One important and common shape in capturing races is called a ladder. The start of a ladder appears on the following board.
Notice that black can atari the two white stones in two ways, with a move at E5 or at F4. If black ataris at E5, white can extend their stones, increasing their liberties from one to three, and thereby escape. Think about what will happen if black ataris at F4 instead. White can extend to increase their liberties from one to two, but this gives black the ability to place white in atari again on the next move.
Do you see how this might play out?
Black can keep white on the run with one atari after another! If there are no other stones on the board, they can run all the way to the edge of the board, where black can capture all of the white stones.
If there is any white stone along that path, however, then the ladder is said to be broken. Suppose our starting position actually looked like this.
Now if we run out the ladder, white escapes! Black cannot atari white in one move, and look at all the moves white can make to put black stones in atari! This is one surprising way in which a capturing race on one side of the board can depend on stones on the opposite side of the board. Familiarize yourself with the ladder shape. It is am important one!
Here’s a video from Nick Sibicky all about reading ladders. It’s a longer video (about 40 minutes) and includes some practice problems and their solutions. Pause the video and try the problems for yourself before looking at the solutions!
