user103: I'm curious about this move duo.
I see this quite a bit, and I'm trying to figure out under which conditions that playing Kb1(b8) is normally preferred.
In some openings, such as the Sicilian Dragon, White will castle long followed up by playing Kb1.  In such cases, there are some rather real threats against the castled position there (Bg7 pointing at b2, open c-file with Rc8, active Black Queen).
However, I'm more curious about some of the more quieter positions where there doesn't appear to be any obvious threats.  In some cases, I see players, and even computer programs, play Kb1(b8) immediately.  I've analyzed some of these lines and never seem to find any reason why Kb1(b8) is played.
I've always been of the mind to just leave the King at c1(c8) unless there is some obvious threat.
Why is Kb1(b8) played in some of these positions?  What types of threats can the other player come up with to require Kb1(b8) to be played? 

user109: Kb1 is generally played to

Defend the unprotected a2 pawn (from ...Qa5 for example)
remove the King from the dangerous c file

If none of the above threats occur, Kb1 is not necessary.

user12: There are two main reasons to play Kb1(b8):

Prophylaxis - Kb1 does many things, xaisoft's answer touches on some of them.  On c1, the king is very exposed to checks on the c1-h6 diagonal.  This means that a queen could check on g5 and pick off a loose piece on h4.  Or similarly, if black plays ...Bh6, the white king will be checked, or worse, a piece could be pinned to the king.  In most openings where white plays O-O-O, the center and kingside pawns are being thrown forward to try and mate black, so avoiding diagonal checks is almost always useful.  Some other reasons that Kb1 is useful are:

Clearing c1 for a rook to help defend c2
Protecting the a2 pawn
Preparing to play c3 or b3 to blunt an attack along the c or b file (although c3 is almost always wrong once O-O-O has been played)

A Waiting Move - Since we have already established that Kb1 is useful, it is also important to note that there are very few drawbacks to the move.  Other moves might not be so good in the position though.  For example, in the Yugoslav Attack of the Dragon, in current theory white will play 12. Kb1 instead of 12. h4 because white is not yet sure how best to deploy the kingside pawns.  Frequently white will just play g4 and omit h4 to speed up the attack.  


103: <p>I'm curious about this move duo.</p>

<p>I see this quite a bit, and I'm trying to figure out under which conditions that playing <code>Kb1(b8)</code> is normally preferred.</p>

<p>In some openings, such as the Sicilian Dragon, White will castle long followed up by playing <code>Kb1</code>.  In such cases, there are some rather real threats against the castled position there (<code>Bg7</code> pointing at b2, open c-file with <code>Rc8</code>, active Black Queen).</p>

<p>However, I'm more curious about some of the more quieter positions where there doesn't appear to be any obvious threats.  In some cases, I see players, and even computer programs, play <code>Kb1(b8)</code> immediately.  I've analyzed some of these lines and never seem to find any reason why <code>Kb1(b8)</code> is played.</p>

<p>I've always been of the mind to just leave the King at c1(c8) unless there is some obvious threat.</p>

<p>Why is <code>Kb1(b8)</code> played in some of these positions?  What types of threats can the other player come up with to require <code>Kb1(b8)</code> to be played? </p>
 109: <p><code>Kb1</code> is generally played to</p>

<ol>
<li>Defend the unprotected a2 pawn (from <code>...Qa5</code> for example)</li>
<li>remove the King from the dangerous c file</li>
</ol>

<p>If none of the above threats occur, <code>Kb1</code> is not necessary.</p>
 12: <p>There are two main reasons to play <code>Kb1(b8)</code>:</p>

<ol>
<li><p><strong>Prophylaxis</strong> - Kb1 does many things, <a href="https://chess.stackexchange.com/a/472/12">xaisoft's answer</a> touches on some of them.  On c1, the king is very exposed to checks on the c1-h6 diagonal.  This means that a queen could check on g5 and pick off a loose piece on h4.  Or similarly, if black plays <code>...Bh6</code>, the white king will be checked, or worse, a piece could be pinned to the king.  In most openings where white plays O-O-O, the center and kingside pawns are being thrown forward to try and mate black, so avoiding diagonal checks is almost always useful.  Some other reasons that Kb1 is useful are:</p>

<ul>
<li>Clearing c1 for a rook to help defend c2</li>
<li>Protecting the a2 pawn</li>
<li>Preparing to play <code>c3</code> or <code>b3</code> to blunt an attack along the c or b file (although <code>c3</code> is almost always wrong once <code>O-O-O</code> has been played)</li>
</ul></li>
<li><p><strong>A Waiting Move</strong> - Since we have already established that <code>Kb1</code> is useful, it is also important to note that there are very few drawbacks to the move.  Other moves might not be so good in the position though.  For example, in the Yugoslav Attack of the Dragon, in current theory white will play <code>12. Kb1</code> instead of <code>12. h4</code> because white is <em>not yet sure</em> how best to deploy the kingside pawns.  Frequently white will just play <code>g4</code> and omit <code>h4</code> to speed up the attack.  </p></li>
</ol>
