No infinite loops

Ko: the repeat rule

pinch or use +/− · double-tap board

Black to move

1.You're Black — capture the single white stone (tap the empty point it's touching). 2.Now you're White — try to take it right back.

Watch what happens: Black captures one stone, and now the position is set up for White to capture one stone right back — returning the board to exactly where it was. Left unchecked, the two of you could do this forever.

So Go has one more rule, ko(Japanese for "eternity"):

You may not play a move that recreates the board position from your opponent's previous turn. In a ko, the recapture is illegal — you must play somewhere else first. Try it: after you capture, tapping straight back is refused with a "Ko!" notice.

To win a ko you play a ko threat elsewhere — a move big enough that your opponent must answer it.
If they answer, the position has changed, so next turn you're allowed to retake the ko.
Kos turn into tense, board-wide trades — they're one of Go's most distinctive fights.

That's the last rule. All that's left is counting up who surrounded more — scoring, in the final lesson.