user12953: I've noticed that when black goes Nd7 in the KID fiancetto variation, then often he follows up with c6. What is the main reason(s) for this? Especially, I'm asking this question as often the d6 pawn afterwards becomes subject for attack by white. First I thought the reason was mainly defensive to hold white knights away from b5 and maybe even d5, but I don't really see this being such a threat. Then I figured the reason for c6 was do go d5 at some point, but rarely does black indeed play this move, so I'm unsure as to why black usually does go c6 and thus creates this target on d6. 
[FEN ""]
[Event "?"]
[Site "?"]
[Date "?"]
[Round "?"]
[White "?"]
[Black "?"]
[Result "*"]

1.d4 Nf6 2.c4 g6 3.Nc3 Bg7 4.Nf3 O-O 5.g3 d6 6.Bg2 Nbd7 7.O-O e5 8.e4 c6 ( 8...Re8 9.h3 c6)


user12269: 
I'm unsure as to why black usually does go c6 and thus creates this target on d6.

The key point is this: it's not a weak point unless it can be attacked, and white can currently only attack it with one bishop, which is easily defended against. If white tries to attack it further, they can only do so by opening the d-file, which ends up removing the weakness (9. dxe5 dxe5, for example) and also demonstrates the usefulness of 8...c6 (it is covering d5).
As for why black is playing c6 over other options: the position in the centre is in considerable flux, and black doesn't know where they want the rest of their pieces until the centre settles (e.g. if white closes with 9.d5, the rook doesn't want to be on e8 and instead was perfectly fine on f8). 8...c6 is a move that retains black's flexibility and gives white another chance to act decisively.
As previously mentioned, it is very useful if the centre opens with 9.dxe5, but it is also useful if white advances 9.d5, because after 9...c5 white can't take en passant and black gets a structure similar to the Czech-Benoni but with more comfortable development.
Of course, white is free to maintain the tension, continuing the theme of the fianchetto response to the KID.
Edit: in the exd5 exd5 lines, you are possibly wondering about the vulnerablity of d6, with the pawn defending it having advanced to c6. This is an important thing to think about, and I can't answer it from my own experience, but in practice the g7 bishop retreating to f8 is enough to cover it (swapping off the the bad bishop in the process :-) )

user1485: First of all, to sit back and wait isn't a good strategy for black, as white has many useful moves to make: h3, Be3, Qc2, Rfe1, Rad1. If white can get these moves in with black doing nothing, white will be clearly better.
So black has to look for counterplay. Most of black's options involve c6 to activate the queen on the d8-a5 diagonal. If you absolutely don't want to move the c-pawn, then one of the few options left is to play exd4 followed by Nc5 and a5. This is playable, but not popular, as white can place an annoying knight on b5, see Giorgiev vs Ilincic, Belgrade 2000.
Black can also try to play exd4 and Nc5 without a5 (instead Bd7, a6, and so on). The game then can get sharp, but current theory says white is comfortable.
In summary: Black plays c6 to get needed counterplay. The other options to get counterplay aren't quite as attractive.

12953: <p>I've noticed that when black goes Nd7 in the KID fiancetto variation, then often he follows up with c6. What is the main reason(s) for this? Especially, I'm asking this question as often the d6 pawn afterwards becomes subject for attack by white. First I thought the reason was mainly defensive to hold white knights away from b5 and maybe even d5, but I don't really see this being such a threat. Then I figured the reason for c6 was do go d5 at some point, but rarely does black indeed play this move, so I'm unsure as to why black usually does go c6 and thus creates this target on d6. </p>

<pre><code>[FEN ""]
[Event "?"]
[Site "?"]
[Date "?"]
[Round "?"]
[White "?"]
[Black "?"]
[Result "*"]

1.d4 Nf6 2.c4 g6 3.Nc3 Bg7 4.Nf3 O-O 5.g3 d6 6.Bg2 Nbd7 7.O-O e5 8.e4 c6 ( 8...Re8 9.h3 c6)
</code></pre>
 12269: <blockquote>
  <p>I'm unsure as to why black usually does go c6 and thus creates this target on d6.</p>
</blockquote>

<p>The key point is this: it's not a weak point unless it can be attacked, and white can currently only attack it with one bishop, which is easily defended against. If white tries to attack it further, they can only do so by opening the d-file, which ends up removing the weakness (9. dxe5 dxe5, for example) and also demonstrates the usefulness of 8...c6 (it is covering d5).</p>

<p>As for why black is playing c6 over other options: the position in the centre is in considerable flux, and black doesn't know where they want the rest of their pieces until the centre settles (e.g. if white closes with 9.d5, the rook doesn't want to be on e8 and instead was perfectly fine on f8). 8...c6 is a move that retains black's flexibility and gives white another chance to act decisively.
As previously mentioned, it is very useful if the centre opens with 9.dxe5, but it is also useful if white advances 9.d5, because after 9...c5 white can't take en passant and black gets a structure similar to the Czech-Benoni but with more comfortable development.</p>

<p>Of course, white is free to maintain the tension, continuing the theme of the fianchetto response to the KID.</p>

<p>Edit: in the exd5 exd5 lines, you are possibly wondering about the vulnerablity of d6, with the pawn defending it having advanced to c6. This is an important thing to think about, and I can't answer it from my own experience, but in practice the g7 bishop retreating to f8 is enough to cover it (swapping off the the bad bishop in the process :-) )</p>
 1485: <p>First of all, to sit back and wait isn't a good strategy for black, as white has many useful moves to make: <code>h3</code>, <code>Be3</code>, <code>Qc2</code>, <code>Rfe1</code>, <code>Rad1</code>. If white can get these moves in with black doing nothing, white will be clearly better.</p>

<p>So black has to look for counterplay. Most of black's options involve <code>c6</code> to activate the queen on the <code>d8</code>-<code>a5</code> diagonal. If you absolutely don't want to move the <code>c</code>-pawn, then one of the few options left is to play <code>exd4</code> followed by <code>Nc5</code> and <code>a5</code>. This is playable, but not popular, as white can place an annoying knight on <code>b5</code>, see <a href="http://www.chessgames.com/perl/chessgame?gid=1403713" rel="nofollow noreferrer">Giorgiev vs Ilincic, Belgrade 2000</a>.</p>

<p>Black can also try to play <code>exd4</code> and <code>Nc5</code> without <code>a5</code> (instead <code>Bd7</code>, <code>a6</code>, and so on). The game then can get sharp, but current theory says white is comfortable.</p>

<p>In summary: Black plays <code>c6</code> to get needed counterplay. The other options to get counterplay aren't quite as attractive.</p>
