10

I only have about 35 games experience , so please excuse blunders.

I am in the following position. My app (dr wolf) wants me to capture on a3 as indicated by the arrow. However I fail to see either the extreme threat posed by the enemy Knight, or the extreme utility of the enemy vacating the b column, that would justify sacrifice (corr.: exchange) of my bishop. enter image description here

Can anyone explain the strengths of this move?

  • 3
    How would you defend e5 if White was allowed to play Nc4? – Andrew Chin Aug 25 at 20:34
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    @AndrewChin I already have a Knight protecting it and I could additionally move the f pawn – Ludi Aug 25 at 20:43
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    Another, perhaps a little less concrete but deeper point: You are making a move that brings your king to castle with tempo (opponent has to recapture), while pretty much eliminating the option of long castling for White, and he needs at least 3 moves before the king is castled kingside, so you can play against his king in the center with Nf6, d5 etc. or already focus on the weak a-pawns. You should also realise that you give White the half-open b file with Bxa3, but it is not relevant. – B.Swan Aug 25 at 22:21
  • 2
    What is that chess software you are using? – Flux Aug 27 at 8:26
  • IMO it's a bad move, but I'm partial to keeping my bishops. It feels unnatural for me here to take the knight. – Mast Aug 27 at 12:12
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  1. Black wrecks white's pawn structure

  2. Black gains in development since he's trading an undeveloped piece for a developed one.

  3. Although the center is sill fluid, black's dark square bishop is currently a "bad" bishop. in the short run, white's knight is the more active piece.

  4. It removes the support of the c-pawn which opens up some tactical possibilities and also forces white to do something in the center rather than sitting back.

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15

Strong positives:

  1. Damages white's pawn structure.
  2. Paves the way for quick kingside castling.
  3. Prevents Nc4, which would centralize the knight and attack e5.

Mild positives:

  1. Removes a potentially "bad bishop" for a knight that is about to become strong.
  2. c3 pawn loses a defender.
  3. a3 pawn is attackable.

Mild negatives:

  1. Opens b-file for your opponent's rook. :(
  2. Trades a bishop for a knight (static material disadvantage of 0.5 pawns). :(
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  • (8) is highly dependent on the situation and is debatable anyway. In this case, I'd say the knight is worth more than the bishop since it's developed—if it weren't, the trade wouldn't be recommended. Right? – user91988 Aug 27 at 20:02
  • Within the current position, the knight is (soon to be) stronger than the bishop: it will exert more control over important squares. However, positions change over time. Positional considerations are dynamic. A bishop will on average outperform a knight, hence why engines give it more static weight throughout the game. This is something to consider before you make the trade. Will the position change in the near future? Will the knight or bishop be more advantageous in such a position? – Mateen Ulhaq Aug 27 at 20:07
  • It's not .5 more, though. More like .25. – user91988 Aug 27 at 20:21
8

While some books give a slightly higher numeric value for the bishop than the knight, an exchange of bishop vs. knight is not regarded as a sacrifice. It depends on the exact position if a bishop or a knight is stronger. So this is simply an exchange.

Bxa3 makes the white pawn structure worse. White gets an isolated double pawn, which is bad news for white. Bxa3 also prevents the knight from Nc4, where it could put some pressure on e5. The negative: black gives away his bishop pair which is advantageous in a lot of positions (but not in all).

My engines (Leela chess zero and Stockfish) agree with Bxa3 but don’t agree with some other white moves. Don’t believe in engine moves, especially from weaker engines (or weaker preferences of strong engines).

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  • 1
    Thanks for the helpful and important correction of nomenclature – Ludi Aug 25 at 20:46
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    @Ludi Also note that ...Bxa3 will help you accelerate your development. You're one move closer to castle and White will have to do something when you attack the a3 pawn with ..Qe7 or ...Qd6 – David Aug 25 at 22:54
4

The thing about loosening White's pawn structure, specifically, is that Black is enabled to develop ...Qd8-e7 or ...Qd8-d6 as a threat, when White's replies a3-a4 or Qd1-c1 don't help White.

Good chess is a matter of introducing unused force with threats.

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