2 
[FEN ""]
[STARTPLY "8"]

1. e4 e5 2. Nf3 d6 3. d4 Nc6 4. c3 Nf6 5. Bg5 h6 6. Bxf6 gxf6 7. Bb5 a6 8. Ba4 b5 9. Bb3 a5 10. O-O b4 11. a3 bxc3 12. Nxc3 exd4 13. Nxd4 Ne5 14. Re1 h5 15. h3 d5 16. Nxd5 f5 17. f3 Bc5 18. Ba4+ Bd7 19. Bxd7+ Qxd7 20. Nf6+ Kd8 21. Nxd7 Kxd7 22. Qc1 Bxd4+ 23. Kh2 Bf2 24. Rd1+ Ke7 25. Qxc7+ Ke6 26. Rd6# 1-0

I do not clearly understand the suggestion by Lichess computer analysis. What is the problem with the move 5.Bg5 if there is one ? Lichess computer analysis suggests me that 5.Bb5 was better, but my idea behind Bg5 was to defend e4 pawn and if you go for Bb5 you sacrifice the pawn and I do not see what compensation I have for it (I played for white). Thanks in advance.

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5
  • 1
    I completely fail to understand how you can make a disgusting move like 17. f3 and then ask a banal question about 5. Bg5.
    – Brian Towers ♦
    Jun 23 '18 at 15:47
  • 2
    @Brian Towers His question wasn't about 17.f3. And if you're going to criticize a move, back it up with a helpful explanation.
    – Inertial Ignorance
    Jun 23 '18 at 22:46
  • In general, don't overestimate engine evaluations in the opening. At this stage, the potential of the game (variation tree) is essentially still infinite. The evaluation number you see evaluates the position that the engine thinks will arise if both sides play optimally for the next X moves. But chances are pretty good that you are not going to hit that exact variation anyways.
    – Annatar
    Jun 25 '18 at 10:59
  • Case in point: The cloud evaluation gives +0.9 for 5. Bb5 at depth 28 before you play it, but only +0.4 at depth 21 after you play it out in the analysis. Apparently, more than half of the white advantage only manifests somewhere between depth 21 and 28. But most likely you or your opponent will deviate from the engine line in move 6 or 7 already, so whatever.
    – Annatar
    Jun 25 '18 at 11:03
  • (Of course, I am not going to say that engine evaluation is completely useless. It just has to be treated with caution.)
    – Annatar
    Jun 25 '18 at 11:06
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Both 5.Bb5 and 5.Bg5 are reasonable here. Lichess wasn't running multiple move analysis, but it didn't mean the move 5.Bg5 was bad.

5.Bb5 doesn't lose a pawn as 5...Nxe4 6.dxe5 dxe5 7.Qxd8+ Kxd8 8.Bxc6 bxc6 9.Nxe5 is better for White.

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3

5.Bg5 isn't that bad of a move, but it's not best because you have to give up your dark-squared bishop after 5...h6. Since some of your dark squares are weak (e.g., f4 and d4), keeping this bishop is necessary to defend the squares.

5.Bb5 is a better move because you're threatening to play 6.d5 and 6.dxe5 (these threats stop Black from taking on e4). If Black answers with 5...Bd7, you can go into this line: 6.d5 Ne7 7.Bxd7 Qxd7 8.Nbd2. Now you've got the e4-pawn protected, and have exchanged off your bad light-squared bishop for Black's good light-squared bishop.

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