user780: This happens sometimes when you're playing a person, more often when playing the computer it seems. The lichess Android app for e.g., on the easiest setting, always follows Nc3 this way (usually following on the first variation below).
[FEN ""]
1. e4 e5 2. Nc3 Bb4 3. a3 ( 3... Bxc3 4. bxc3 Nf6 5. Rb1) (3... Ba5 4. b4 Bb6)  

Every time I see this played I wonder what black is trying to gain here. He either ends exchanging the bishop for my knight (and giving my rook a good open file) or locking his bishop in one corner, neither seems particularly useful.  
Am I missing something? 

user4145: This is an opening reminiscent of the Ruy Lopez. Black has several goals when playing 2... Bb4:

Move the bishop out of the way in order to prepare to castle.
Set up a potential knight-to-king pin, preventing white from advancing the important central d-pawn, which white will usually want to do in order to gain better control of the center.
Attack the knight that is defending the e-pawn—this isn't as immediately productive as it is when played by white, as black has no way to subsequently capture it; however, it's still a threat.
If black does trade the bishop for the knight, recapturing it will ruin white's pawn structure. Recapturing with the b-pawn is a bad idea (allowing 2... Bb4 3. a3 Bxc3 4. bxc3? d5 5. exd5 Qxd5 giving black better control of the center), and hence white is forced to move the important d-pawn aside.


user291: It is indeed a little silly to play ...Bb4 on move 2 here. That said, it often gets played a little later, and the main idea is to pin the knight on c3 (after the d-pawn has moved), so that it no longer protects the pawn on e4, which can now be attacked by a knight on f6, say.
In the particular line you gave, by the way, the bishop is doing just fine on b6. You spent time kicking it to a place where it's aiming right at the weak pawn on f2 and beyond to your king on g1 once you castle.

780: <p>This happens sometimes when you're playing a person, more often when playing the computer it seems. The lichess Android app for e.g., on the easiest setting, <em>always</em> follows Nc3 this way (usually following on the first variation below).</p>

<pre><code>[FEN ""]
1. e4 e5 2. Nc3 Bb4 3. a3 ( 3... Bxc3 4. bxc3 Nf6 5. Rb1) (3... Ba5 4. b4 Bb6)  
</code></pre>

<p>Every time I see this played I wonder what black is trying to gain here. He either ends exchanging the bishop for my knight (and giving my rook a good open file) or locking his bishop in one corner, neither seems particularly useful.  </p>

<p>Am I missing something? </p>
 4145: <p>This is an opening reminiscent of the <a href="http://en.wikipedia.org/wiki/Ruy_Lopez" rel="nofollow">Ruy Lopez</a>. Black has several goals when playing 2... Bb4:</p>

<ol>
<li><p>Move the bishop out of the way in order to prepare to castle.</p></li>
<li><p>Set up a potential knight-to-king pin, preventing white from advancing the important central d-pawn, which white will usually want to do in order to gain better control of the center.</p></li>
<li><p>Attack the knight that is defending the e-pawn—this isn't as immediately productive as it is when played by white, as black has no way to subsequently capture it; however, it's still a threat.</p></li>
<li><p>If black does trade the bishop for the knight, recapturing it will ruin white's pawn structure. Recapturing with the b-pawn is a bad idea (allowing 2... Bb4 3. a3 Bxc3 4. bxc3? d5 5. exd5 Qxd5 giving black better control of the center), and hence white is forced to move the important d-pawn aside.</p></li>
</ol>
 291: <p>It is indeed a little silly to play ...Bb4 on move 2 here. That said, it often gets played a little later, and the main idea is to pin the knight on c3 (after the d-pawn has moved), so that it no longer protects the pawn on e4, which can now be attacked by a knight on f6, say.</p>

<p>In the particular line you gave, by the way, the bishop is doing just fine on b6. You spent time kicking it to a place where it's aiming right at the weak pawn on f2 and beyond to your king on g1 once you castle.</p>
