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The Ko Rule in Go: Why You Can't Immediately Recapture

⏱ Read ~6 min · Updated July 2026 · Part of: Learn Go

Ko (劫) has exactly one purpose: stop the same capture from repeating forever. It's the only rule in Go that forbids a move — and once you've seen the picture it prevents, you never forget it.

English中文Plain meaning
ko劫／打劫after capturing one stone, the opponent can't recapture immediately
ko threat劫材a forcing move elsewhere, used to gain time
ko fight打劫the back-and-forth battle over a ko
sente先手initiative — the opponent must respond
gote後手loss of initiative — you respond

1. The infinite loop ko prevents

Picture this shape: a black stone and a white stone, each down to one liberty, either one able to capture the other.

    .  B  W  .
    B  W  .  .
    .  .  .  .
    .  .  .  .

With no rule, this loops forever:

  1. Black captures the white stone. ♻️ Black fills white's last liberty; white is removed.
  2. White recaptures the black stone. ♻️ White plays on the same point, filling black's last liberty; black is removed.
  3. Black recaptures again… The board returns to the start, forever unchanged.

The ko rule breaks the loop in one sentence: after you capture one stone in a ko, the opponent cannot immediately recapture that same stone. They must first play elsewhere.

🛑 Analogy: two people fighting over one parking spot. If they could grab the keys back instantly, they'd fight forever. The rule is "let go first, go do something else, then come back" — you must play elsewhere (tenuki) before you can come back and recapture.

2. What a ko looks like (recognize this shape)

🔁 A ko is two single stones that just captured each other's neighboring point — a tight 1-for-1. Its features:

When you see this shape, remember: the side that just got captured must first play elsewhere (tenuki) before taking the stone back.

3. Ko threats — how a ko actually plays out

Since the recapturing side must play elsewhere first, they play a ko threat — a forcing move elsewhere on the board that the opponent should respond to. Once the opponent responds, the recapturing side is free to retake the ko.

This back-and-forth is the ko fight:

Black captures ko → White plays ko threat (forcing elsewhere) → Black responds → White recaptures ko
→ Black plays ko threat → White responds → Black recaptures → … (until one side decides it's not worth fighting anymore)

🎯 Who wins the ko? The side with more (or bigger) ko threats usually wins — because the opponent eventually runs out of threats big enough to force a response, and has to let you take the ko. So "hoarding ko threats" during quiet moments is ammunition for later fights.

4. Sente and gote — the engine of ko threats

Ko threats work because of sente (先手) and gote (後手):

A good ko threat is sente — it forces a response, buying you the right to recapture the ko. If your threat is gote (ignorable), you've wasted a move and the opponent wins the ko outright.

The same sente/gote concept runs through the entire game; ko is just where beginners first meet it.

5. The 4 most common ko mistakes for beginners

  1. Not knowing "no immediate recapture." This is the entire ko rule — you just captured a stone in the ko, the opponent cannot recapture on that same point right now. Beginners often think "if I can take it, I take it" — that's a rule violation.
  2. Fighting a ko with no threats. If you have few ko threats and fight anyway, you're handing moves to the opponent — they ignore your (gote) threats and win the ko. Count "do I have threats big enough?" before fighting.
  3. Misjudging superko (advanced, big board). On 19×19, the stricter "superko" rule forbids repeating any board position that has ever appeared (not just immediate recapture). On 4×4 you only meet the simple "no immediate recapture" version — but be aware of superko on the big board.
  4. Letting kos go without valuing them. Sometimes a ko is worth a lot (decides a group's life or death) and is worth spending threats to fight. Don't let every ko go by default — calculate whether it's worth the fight.

6. Ko on the 4×4 board — a clean teaching tool

On the 4×4 board, kos appear fast and resolve fast, because the board is small and there's little room for big threats elsewhere. This makes ko a clean teaching tool: you see the loop risk, you see the ko-threat mechanism, and the fight ends quickly. The brainGO engine uses the "simple ko" rule (no immediate recapture) — that's all you'll meet on 4×4; the full superko rule matters only on bigger boards.

How to train it

The ko rule clicks once you've watched the loop happen once and then seen the rule break it.

👉 Play brainGO — meet your first ko

FAQ

Why can't you immediately recapture in a ko?

Because if it were allowed, black and white would infinitely recapture each other, the board would never change, and the game would never end. The ko rule exists to break this loop — after capturing a stone, the opponent must first play elsewhere before recapturing.

What is a ko threat and how do you use it?

A ko threat is "a forcing move elsewhere" — the kind the opponent must respond to (sente). When you're forbidden from recapturing, you first play a threat elsewhere; after the opponent responds, you're free to recapture the ko. The more and bigger your threats, the more likely you win the ko.

What's the difference between ko and superko?

Ko (simple) only forbids "immediate recapture on the same point." Superko (full) forbids "repeating any board position that has ever appeared" — it's stricter. 4×4 only needs simple ko; 19×19 official rules use superko.

When should I spend a ko threat?

When the ko is worth enough (deciding life/death, a big group, or the game) to justify spending a move to fight for it. If the ko is small and your threats are precious, save them for elsewhere. Calculating "is it worth it" is the advanced skill.

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