Answers.Body: <p>I'd argue you never want to play such a bishop sacrifice if your opponent is willing to take it. </p>
  
  <p>Therefore, I think your question is rather “when can I take the undefended <code>♟ a7</code>”.</p>
  
  <p>To answer this, I'd say you need</p>
  
  <ul>
  <li><p>Either a defense on the bishop (with a rook or a knight)</p></li>
  <li><p>Or a crushing attack if you can sacrifice it. A queen around, or a rook coming on the open c-file, with pieces supporting that are mandatory. If the king can't easily escape to <code>e8</code>, that's better.</p></li>
  <li><p>Nothing better to do. That's just a far pawn, and it takes 2 tempi to take eat if you want to save your bishop. Your opponent need not take bigger of an advantage in the meantime, that you could defend from before taking the pawn.</p></li>
  </ul>
  
Answers.ContentLicense: CC BY-SA 3.0
Answers.CreationDate: 2012-12-06T15:31:22.837Z
Answers.Text: In other words, make sure you have a material advantage.
Answers.UserDisplayName: xaisoft
Answers.ContentLicense: CC BY-SA 3.0
Answers.CreationDate: 2012-12-06T15:09:55.857Z
Answers.LastActivityDate: 2012-12-06T15:09:55.857Z
Answers.UserDisplayName: Nikana Reklawyks
Body: <p>Though queenside castling is less common, it is important enough to study. If your opponent has castled queenside, sometimes you have an opportunity to expose their King by sacrificing your Bishop for 2 pawns, as showed below. (I have removed other pieces from the board to avoid distraction.)</p>
  
  <pre><code>[FEN "2kr4/ppp5/8/8/8/4B3/8/4K3 w - - 0 1"]
  
  1.Bxa7 b6 2.Bxb6 cxb6 
  </code></pre>
  
  <p>What factors should one consider before deciding to offer this sacrifice? To put it another way, in what positions is this sacrifice favourable and in what positions would it be unfavourable?</p>
  
  <p><strong>Edit</strong> As per comment from <a href="https://chess.stackexchange.com/users/109/xaisoft">xaisoft</a>, I am posting a board with other pieces. </p>
  
  <pre><code>[FEN "rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1"]
  
  1.e4 e5 2.d4 d6 3.Bc4 Nf6 4.Nf3 Nc6 5.dxe5 dxe5 6.Nxe5 Nxe5 7.Qe2 Bd7 8.Nc3 Qe7 9.Be3 O-O-O 10.Bxa7 b6 11.Bxb6 cxb6
  </code></pre>
  
Comments.ContentLicense: CC BY-SA 3.0
Comments.CreationDate: 2012-12-06T14:12:40.620Z
Comments.Text: Ofter enough, your opponent will defend his pawn rather than trying to trap the bishop, once he has castled.
Comments.UserDisplayName: Nikana Reklawyks
Comments.ContentLicense: CC BY-SA 3.0
Comments.CreationDate: 2012-12-06T14:30:28.977Z
Comments.Text: I agree, but if they choose to do that, you can simply "abort" the sacrifice and retract your bishop. Here I want to analyse the variation where they choose to play along and accept the sacrifice.
Comments.UserDisplayName: Masked Man
Comments.ContentLicense: CC BY-SA 3.0
Comments.CreationDate: 2012-12-06T15:31:46.533Z
Comments.Text: @ap. - Would you mind posting a board that has all the pieces?
Comments.UserDisplayName: xaisoft
Comments.ContentLicense: CC BY-SA 3.0
Comments.CreationDate: 2012-12-06T16:04:45.747Z
Comments.Text: @xaisoft Well, the board was only meant to illustrate the pattern. I did not put the other pieces lest the answers get distracted by that specific position. I want to learn the general principles to be observed in such cases. If you insist, I will put the other pieces too.
Comments.UserDisplayName: Masked Man
Comments.ContentLicense: CC BY-SA 3.0
Comments.CreationDate: 2012-12-06T16:11:05.507Z
Comments.Text: @ap. - You can keep the original board you posted, I was talking about adding a second board with more pieces.
Comments.UserDisplayName: xaisoft
Comments.ContentLicense: CC BY-SA 3.0
Comments.CreationDate: 2012-12-07T22:22:47.287Z
Comments.Text: In the position you post on the second board, Black is already a piece up, he should exchange material and win with his knight advantage. `10... Nxc4 11. Qxc4 Qb4 -+`. Regarding my answer, that's a case where White *has* better to do, and shouldn't take the undefended pawn.
Comments.UserDisplayName: Nikana Reklawyks
Comments.ContentLicense: CC BY-SA 3.0
Comments.CreationDate: 2014-03-20T02:44:25.277Z
Comments.Text: Am I misreading things?  On your second board, after `10...b6?`, why `11. Bxb6`?  Why not `11. Ba6#`?  I think the danger to white from `Bxa7` isn't `...b6`, but rather the fact that it costs time.
Comments.UserDisplayName: supercat
Comments.ContentLicense: CC BY-SA 3.0
Comments.CreationDate: 2014-03-20T04:38:44.997Z
Comments.Text: @supercat *Read the question* buddy. It is just an example I came up with to illustrate my question, specifically because I was asked for an example with the full set of pieces (as I have mentioned in the question details). Here I am not concerned about the fastest way to win from this particular position. If you can create a better example illustrating the question, please feel free to edit.
Comments.UserDisplayName: Masked Man
Tags: <analysis><tactics><sacrifice>
Title: Sacrificing Bishop for Two Pawns against Queenside Castling
ThreadText: # Sacrificing Bishop for Two Pawns against Queenside Castling
  
  (Asked by: username_0 on 2012-12-06)
  
  Though queenside castling is less common, it is important enough to study. If your opponent has castled queenside, sometimes you have an opportunity to expose their King by sacrificing your Bishop for 2 pawns, as showed below. (I have removed other pieces from the board to avoid distraction.)
  
  ```
  [FEN "2kr4/ppp5/8/8/8/4B3/8/4K3 w - - 0 1"]
  
  1.Bxa7 b6 2.Bxb6 cxb6 
  
  ```
  
  What factors should one consider before deciding to offer this sacrifice? To put it another way, in what positions is this sacrifice favourable and in what positions would it be unfavourable?
  
  **Edit** As per comment from [username_2](https://chess.stackexchange.com/users/109/username_2), I am posting a board with other pieces.
  
  ```
  [FEN "rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1"]
  
  1.e4 e5 2.d4 d6 3.Bc4 Nf6 4.Nf3 Nc6 5.dxe5 dxe5 6.Nxe5 Nxe5 7.Qe2 Bd7 8.Nc3 Qe7 9.Be3 O-O-O 10.Bxa7 b6 11.Bxb6 cxb6
  
  ```
  
  **Tags:** analysis, tactics, sacrifice
  
  > **Comments:**
  > **username_1**: Ofter enough, your opponent will defend his pawn rather than trying to trap the bishop, once he has castled.  
  > **username_0**: I agree, but if they choose to do that, you can simply "abort" the sacrifice and retract your bishop. Here I want to analyse the variation where they choose to play along and accept the sacrifice.  
  > **username_2**: @ap. - Would you mind posting a board that has all the pieces?  
  > **username_0**: @username_2 Well, the board was only meant to illustrate the pattern. I did not put the other pieces lest the answers get distracted by that specific position. I want to learn the general principles to be observed in such cases. If you insist, I will put the other pieces too.  
  > **username_2**: @ap. - You can keep the original board you posted, I was talking about adding a second board with more pieces.  
  > **username_1**: In the position you post on the second board, Black is already a piece up, he should exchange material and win with his knight advantage. `10... Nxc4 11. Qxc4 Qb4 -+`. Regarding my answer, that's a case where White *has* better to do, and shouldn't take the undefended pawn.  
  > **username_3**: Am I misreading things?  On your second board, after `10...b6?`, why `11. Bxb6`?  Why not `11. Ba6#`?  I think the danger to white from `Bxa7` isn't `...b6`, but rather the fact that it costs time.  
  > **username_0**: @username_3 *Read the question* buddy. It is just an example I came up with to illustrate my question, specifically because I was asked for an example with the full set of pieces (as I have mentioned in the question details). Here I am not concerned about the fastest way to win from this particular position. If you can create a better example illustrating the question, please feel free to edit.  
  
  ---
  
  ## 1 answers
  
  ### Answer by username_1 on 2012-12-06. Score: 5 | Accepted answer
  
  I'd argue you never want to play such a bishop sacrifice if your opponent is willing to take it.
  
  Therefore, I think your question is rather “when can I take the undefended `♟ a7`”.
  
  To answer this, I'd say you need
  
  * Either a defense on the bishop (with a rook or a knight)
  * Or a crushing attack if you can sacrifice it. A queen around, or a rook coming on the open c-file, with pieces supporting that are mandatory. If the king can't easily escape to `e8`, that's better.
  * Nothing better to do. That's just a far pawn, and it takes 2 tempi to take eat if you want to save your bishop. Your opponent need not take bigger of an advantage in the meantime, that you could defend from before taking the pawn.
  
  > **Comments:**
  > **username_2**: In other words, make sure you have a material advantage.  
  
  ---
  
  