user3237: I was just wondering, if as Black I normally play the Caro-Kann and the Slav (but I slightly prefer the Caro-Kann), is it a good idea to answer 1. d4 with 1... c6!?
I looked in an opening database, but it looks like 1... c6 is extremely rare and is only played 0.3% of the time.
What's wrong with it? Doesn't it just transpose into either a Slav or a Caro-Kann?
Does it have a psychological effect on weak opponents? Maybe they will be a little surprised, and thus they will take slightly more time to make their second move, and so it would be an effective subterfuge especially in blitz.

user17: C6 is a good move to "throw people off."
Sometimes, you will get e4 in return and go into your Caro-Kann.
Most of the time, the reply will be c4. But even the Slav has some advantages. Specifically, there may later be a chance to play d5xc4, queens gambit accepted, with some chance of holding the pawn. 

user8291: According to current chess opening strategy 1... c6 is not considered very useful if White continues with the London, Torre or Colle Systems, unless you want to follow with some sort of King's Indian or Modern Defense setup, which does not seem to be your case from your question and opening preferences, so as a Slav player I'd rather play 1... d5 vs 1. d4. On the other side, the London, Torre and Colle aren't considered particularly challenging for Black from a theoretical point of view, so at the end of the day it is a matter of personal choice and tastes. 

user2429: You are absolutely right to think that it is a good idea to answer 1.d4 with 1...c6, especially if you like the Caro-Kann more than the Slav. Do not let the low percentage for the move bother you. I also checked in chessbase's Big Database and found that it is played about 4.5% of the time. However, that is not because the move is weak, but because the overwhelming majority of the players prefer to play 1...Nf6 or 1...d5, because not everyone wants to play the Caro Kann. 
1...c6 may have a psychological effect on weak opponents and if they hesitate to immediately play 2. e4 or 2. c4, then you know that you caught them off guard for a moment. Gary Kasparov himself played 1...c6 in a blitz game. 
Hristos Banikas vs Garry Kasparov 0-1, 2001

user924: I don't think it's a good idea. Firstly, no 1.d4 player will go for 2.e4, unless they are also 1.e4 players and really, really good at playing against the Caro-Kann. 
More importantly, you may like to play the Slav against d4+c4, and the Caro against d4+e4, but so far white has only played 1.d4. You lose options in case white doesn't follow up with a quick c4 or e4.
Mostly that's lines like the London (say 2.Nf3 and 3.Bf4) or the Colle (with Nf3, e3, Bd3, possibly b3 and Bb2, etc). Against both of those, black's best chance to get an active game are with a quick ...c5, but if you do that you've already given up a tempo.
So there is a downside, and zero upside.

3237: <p>I was just wondering, if as Black I normally play the Caro-Kann and the Slav (but I slightly prefer the Caro-Kann), is it a good idea to answer 1. d4 with 1... c6!?</p>

<p>I looked in an opening database, but it looks like 1... c6 is extremely rare and is only played 0.3% of the time.
What's wrong with it? Doesn't it just transpose into either a Slav or a Caro-Kann?</p>

<p>Does it have a psychological effect on weak opponents? Maybe they will be a little surprised, and thus they will take slightly more time to make their second move, and so it would be an effective subterfuge especially in blitz.</p>
 17: <p>C6 is a good move to "throw people off."</p>

<p>Sometimes, you will get e4 in return and go into your Caro-Kann.</p>

<p>Most of the time, the reply will be c4. But even the Slav has some advantages. Specifically, there may later be a chance to play d5xc4, queens gambit <em>accepted,</em> with some chance of holding the pawn. </p>
 8291: <p>According to current chess opening strategy <strong>1... c6</strong> is not considered very useful if White continues with the London, Torre or Colle Systems, unless you want to follow with some sort of King's Indian or Modern Defense setup, which does not seem to be your case from your question and opening preferences, so as a Slav player I'd rather play <strong>1... d5</strong> vs <strong>1. d4.</strong> On the other side, the London, Torre and Colle aren't considered particularly challenging for Black from a theoretical point of view, so at the end of the day it is a matter of personal choice and tastes. </p>
 2429: <p>You are absolutely right to think that it is a good idea to answer <strong>1.d4</strong> with <strong>1...c6</strong>, especially if you like the Caro-Kann more than the Slav. Do not let the low percentage for the move bother you. I also checked in chessbase's Big Database and found that it is played about 4.5% of the time. However, that is not because the move is weak, but because the overwhelming majority of the players prefer to play <strong>1...Nf6</strong> or <strong>1...d5</strong>, because not everyone wants to play the Caro Kann. </p>

<p><strong>1...c6</strong> may have a psychological effect on weak opponents and if they hesitate to immediately play <strong>2. e4</strong> or <strong>2. c4</strong>, then you know that you caught them off guard for a moment. Gary Kasparov himself played <strong>1...c6</strong> in a blitz game. </p>

<p><a href="http://www.chessgames.com/perl/chessgame?gid=1173479" rel="nofollow">Hristos Banikas vs Garry Kasparov 0-1, 2001</a></p>
 924: <p>I don't think it's a good idea. Firstly, no 1.d4 player will go for 2.e4, unless they are also 1.e4 players and really, really good at playing against the Caro-Kann. </p>

<p>More importantly, you may like to play the Slav against d4+c4, and the Caro against d4+e4, but so far white has only played 1.d4. You lose options in case white doesn't follow up with a quick c4 or e4.</p>

<p>Mostly that's lines like the London (say 2.Nf3 and 3.Bf4) or the Colle (with Nf3, e3, Bd3, possibly b3 and Bb2, etc). Against both of those, black's best chance to get an active game are with a quick ...c5, but if you do that you've already given up a tempo.</p>

<p>So there is a downside, and zero upside.</p>
