Placing a stone middle of four enemy stones


#1

Hi folks. I’m trying to learn the game, but I keep getting stuck in logic-traps on the basics.

For example, in the OGS go tutorial problem here:

  1. I place a black stone at E2.
  2. white responds at E4.
  3. The tutor tells me to place a black stone at D3… but I thought that was illegal, since that point is completely surrounded by white. So I do anyway, and win a bunch of white stones.

What is the rule that allows this move? How can I tell when placing a black stone in the middle of four whites is allowed, and when it is not?

Kurt


#2

A move generally consists of placing a stone, then removing hostile stones without liberties. If the stone has no liberties after this process the move is illegal (in rulesets that forbid suicide). The tutorial move is legal because the stone has liberties after hostile stones are removed.


#3

This is such a frequent and natural question, I wonder it it should be explicitly addressed at that place in the tutorial?


#4

Still doesn’t compute: As I place the stone, for the instant, NEITHER my black nor the white has any liberties.

So is the rule more properly stated: “you can place a stone in a “surrounded point” as long as that placement IS ALSO the last liberty that the opposing side has for another group?”


#5

GreenAsJade - yes please. and I still don’t understand the answer. As I place my white stone, NOBODY has any liberties… and more than that, it resulted from my placing my stone in a spot with no liberties in the first place.


#6

Yes. The chess analogy of your contention is: “Every square can only be occupied by a single piece, but if I take your pawn with my rook, in the instant before I take your pawn off the board, two pieces occupy the same square.”


#7

Thanks folks!


#8

I’m sorry for being unclear. There is no instant for which neither black nor white has any liberties, because you remove hostile stones as part of the process of making a move. @smurph 's chess analogy is a good one. It’s like that.


#9

The second step (it’s still your move) is to remove the stones of the other color that have no liberties. In your case, you would remove D3 from the board. Only then, you would look at your own color and see that your stones now have one liberty, which confirms the validity of your move.

Now same routine when black plays back at D3.


#10

I will chase up getting that point made in the tutorial.

As I said, it is a common and natural question, which I’ve bumped into often in teaching games.


#11

Do you think that this is the lesson in which to make this point?

It seems so.