第7课：打劫 | Lesson 7: Ko Rule

What Is Ko?

In Go, a special situation sometimes arises where both players could capture back and forth at the same point endlessly. Without any restriction, the game would never finish. To solve this problem, Go has the ko rule (also called simply "ko").

The word "ko" comes from Japanese, meaning "eternity." The ko rule is one of the most elegant rules in Go. It not only prevents infinite loops but also creates one of the most exciting tactical elements in the game -- the ko fight.

How Ko Forms

Let us walk through an example to understand how a ko situation arises.

Step 1: The Initial Position

The diagram below shows a typical ko shape. Notice how the black and white stones interlock:

The green dot marks the last liberty of the white stone. If Black plays on the green dot, it captures that white stone.

Step 2: Black Captures the White Stone

Black plays on the green dot position and captures the white stone. But notice -- now the newly placed black stone itself has only one liberty:

Now the situation is reversed -- White can capture the black stone at the point where the white stone was just removed. If White immediately captures back, the board returns to the Step 1 position, and then Black could capture again... creating an infinite loop!

Why the Ko Rule Is Necessary

Imagine Go without the ko rule:

1. Black captures one white stone
2. White immediately captures back
3. Black captures again
4. White captures back again...

This cycle would never end, making the game impossible to complete. The ko rule breaks this cycle by requiring that you "cannot immediately recapture."

What Are Ko Threats?

Since you cannot immediately recapture, the player who lost the ko must play elsewhere first. This move is called a "ko threat". A good ko threat is a move that your opponent must respond to -- this way you earn the right to recapture the ko on your next turn.

The Logic of Ko Threats

• Black captures the ko (takes one white stone)
• White cannot immediately recapture, so White plays a ko threat elsewhere
• If the threat is big enough, Black must respond, and White can recapture the ko next turn
• If the threat is not big enough, Black ignores it and fills the ko (permanently resolving the ko position)

Ko Fights -- The Most Intense Battles in Go

When a ko is very important (for example, it determines the life or death of a large group), both sides will fight fiercely over it. This is called a ko fight.

The ko fight process:

1. Player A captures the ko
2. Player B plays a ko threat elsewhere (threatening Player A)
3. If Player A responds to the threat, Player B recaptures the ko
4. Player A plays a ko threat elsewhere (threatening Player B)
5. The cycle continues...

Ko fights often involve calculations across the entire board -- you need to judge which ko threats are big enough and which can be ignored. This is one of the most complex and exciting tactical elements in Go.

How Beginners Should Handle Ko

When you encounter a ko, do not panic. Keep these points in mind:

• No immediate recapture: This is the most basic rule, and Cute Go enforces it automatically
• Evaluate the ko's value: How much does this ko affect the game? If not much, consider giving it up
• Look for ko threats: Find moves elsewhere that your opponent must respond to
• Consider filling the ko: If the ko fight seems unfavorable, you can simply fill the ko point (called "connecting the ko"). You lose one move, but avoid greater risk

Positional Superko

A stricter version of the ko rule is called "Positional Superko." In simple terms: the entire board position cannot repeat any position that has appeared earlier in the game.

The basic ko rule only prohibits immediately recapturing a single stone. The superko rule is more comprehensive, preventing all possible board repetitions, including some rare multi-step cycles.

As a beginner, you only need to remember the most basic rule: after a stone is captured, you cannot immediately recapture it. More complex situations are extremely rare in actual play.