I wasn't able to play much last month and so I decided to play a game today.

In this game I payed against a 6k and since I am 7k it was 0.5 komi and I'm Black.
I had the feeling that the Opening was good for me and I only needed to play a bit defensive to keep it that way.

I know I made a big mistake at the bottom during the end (the ko that happend), also I made a mistake earlier and my group got cut in the middle of the board.
But other then that I don't really think I made such big mistakes, of course I did but I just don't know what^^

I would really appreciate if someone can tell me what could have done better.

@ O3 is huge - this is such a vital shape to remember.
@ C15 fixes your cutting weaknesses, and is likely to do so in sente.
@ P15 creates a two-pronged attack without strengthening either side - this is normally better when you already have pincered both groups.

Yes, @ O3 is huge -- you cannot miss this shared vital point for both B and W.
Maybe more later... Gotta run to Go Congress!

As noted above, move 21 at O3 is important. I'm sure that you can see that once someone suggests it. But why did they see it?

Let's back up a bit, and look at the shape, which should suggest it for you. Even if white does not have territory that you can undercut, O3 should occur to you.

The relevant shape is this:

Click Here To Show Diagram Code

[go]$$W
$$. . . . . . . . .
$$. . . . . . . . .
$$. . . . . . . . .
$$. . . . O . . . .
$$. . . X O . . . .
$$. . . . X . . . .
$$. . . . . . . . .
$$. . . . . . . . .
$$-----------------[/go]

Immediately upon seeing that, it should occur to you that if you tenuki, this may happen:

Click Here To Show Diagram Code

[go]$$W
$$. . . . . . . . .
$$. . . . . . . . .
$$. . . . . . . . .
$$. . . . O . . . .
$$. . . X O . . . .
$$. . . 1 X . . . .
$$. . . . . . . . .
$$. . . . . . . . .
$$-----------------[/go]

...which, with a little tewari swapping, can also be seen as this:

Click Here To Show Diagram Code

[go]$$W
$$. . . . . . . . .
$$. . . . . . . . .
$$. . . . . . . . .
$$. . . . 1 . . . .
$$. . . X O . . . .
$$. . . O X . . . .
$$. . . . . . . . .
$$. . . . . . . . .
$$-----------------[/go]

In other words, at move 21, you should see the crosscut. And once you can see it, surely you will recall the proverb about 'extend from a crosscut'. And you can see that in the above diagram is an extension off of the crosscut - good for him, bad for you.

So, once you have visualized the crosscut that may happen if you tenuki at 21, and have observed that it would be bad for you, you have two possibilities:
1) Connect so that there is no crosscut.
2) Make the first extension so that you have the advantage in the crosscut fight if he starts it.

Connecting, though perhaps unenterprising, is a good solid move:

Click Here To Show Diagram Code

[go]$$B
$$. . . . . . . . .
$$. . . . . . . . .
$$. . . . . . . . .
$$. . . . O . . . .
$$. . . X O . . . .
$$. . . 1 X . . . .
$$. . . . . . . . .
$$. . . . . . . . .
$$-----------------[/go]

...and in many positions with this shape it will be the best move. It should make it on to your short list of candidates for the next move.


If you want to extend, there are four possibilites:

Click Here To Show Diagram Code

[go]$$B
$$. . . . . . . . .
$$. . . . . . . . .
$$. . . . . . . . .
$$. . . a O . . . .
$$. . b X O . . . .
$$. . . C X c . . .
$$. . . . d . . . .
$$. . . . . . . . .
$$-----------------[/go]

I won't go into any depth on the four, but note that all four should make it on to your short list. Once you look at 'c', you won't need EdLee or Topazg to tell you about it, because you will have played it yourself.

EDIT: Just for the sake of completeness, note that 'c' does win the crosscut fight for you if he starts it:

Click Here To Show Diagram Code

[go]$$W
$$. . . . . . . . .
$$. . . . . . . . .
$$. . . . . . . . .
$$. . . . O . . . .
$$. . . X O . . . .
$$. . 2 1 X B . . .
$$. . 4 3 . . . . .
$$. . . . . . . . .
$$-----------------[/go]

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Thanks to everyone, and thanks Joaz for that detailed explanation!
I keep that in mind and in fact my first thought was to play 21 at O3 but I thought my right grou is too weak, O3 seemed too aggressive.
But if I look at it now it looks a lot better then what I played.