user549: I was playing chess this afternoon on my mac, as human vs computer. In the position below:
[FEN "6k1/5pp1/2p3bp/pqP1P3/2R3P1/PPQ5/4K3/8 w - - 0 24"]

I tried to place the Rook in front of Black's Queen (b4), but this action was blocked and the Rook remained in the same place. Later I completed the game by making another move.
Why is this so? Why can't I place the Rook in front of Black's Queen? Please explain this.

user17: You wouldn't want to place the rook in front of the queen at b4. Because it can be captured by the pawn on a5.
Even if you do want to, you can't move the rook in front of the queen (or anywhere else), because it is pinned to your king by the queen.
It's fortunate that Rb4 was an illegal move, because it would have been a big mistake. 
You should, instead, move the king to e3, unpinning it, and bringing it closer to your pawns.

user70: The Rook on c4 is pinned against the your King by the black Queen. Thus you can't move it without exposing your King to the Queen's wrath.

549: <p>I was playing chess this afternoon on my mac, as human vs computer. In the position below:</p>

<pre><code>[FEN "6k1/5pp1/2p3bp/pqP1P3/2R3P1/PPQ5/4K3/8 w - - 0 24"]
</code></pre>

<p>I tried to place the Rook in front of Black's Queen (b4), but this action was blocked and the Rook remained in the same place. Later I completed the game by making another move.</p>

<p>Why is this so? Why can't I place the Rook in front of Black's Queen? Please explain this.</p>
 17: <p>You wouldn't <em>want</em> to place the rook in front of the queen at b4. Because it can be captured by the pawn on a5.</p>

<p>Even if you do want to, you <em>can't</em> move the rook in front of the queen (or anywhere else), because it is pinned to your king by the queen.</p>

<p>It's fortunate that <code>Rb4</code> was an illegal move, because it would have been a big mistake. </p>

<p>You should, instead, move the king to e3, unpinning it, and bringing it closer to your pawns.</p>
 70: <p>The Rook on c4 is pinned against the your King by the black Queen. Thus you can't move it without exposing your King to the Queen's wrath.</p>
