Was shared a chess position and reached this position. I didn't understand why Ng5 is a draw. I analyzed h6 and h5 as responses but they fall to the same checkmating theme of rook checkmate on h8. More precisely... Bxh5... Bf2... Rg8... Rg5... Rg4... Rgh4... and threaten checkmate.

1r3rk1/5p1p/1qb1pPp1/3pP3/p1pP4/PpP2N1R/1Pn1B2P/3RB2K w - - 1 3

Can someone enlighten me on why Ng5 isn't winning?

  • If this is the same post from /r/chess, I believe the answer is black can sacrifice their queen on c3 and breakthrough on the queenside before you have time to checkmate them – NoseKnowsAll Jan 24 at 2:15
  • So assuming Qa5 then Rxh7. After Qxc3, I didn't find any breakthrough after Bxc3. Maybe black can then play Nxa3 but I don't have to react and just play Rg7# Kh8, followied by Rg1... Rg3... Rh3#. – edit_profile Jan 24 at 5:00
  • Looks like black ph5 can hold for the draw. did not analyse it to the end. comment if you think white can stil win. – Phischmaster Jan 24 at 22:15

When I feed the position to Stockfish (12), it's first impression (depth<20) is that it is won for Black (evaluation at around -8.0). But that's probably just counting material.

After some calculation, the evaluation changes to 0.0 - for four different first moves by Black: 1...h6, 1...h5, 1...Nxa3 and 1...Ne3 (still holds at depth 43).

That's probably enough options to claim that White cannot bust them all and win.

Example analysis of 1. Ng5 h5 2. Bxh5 Nxa3 (0.0 by Stockfish at depth 41)

  • 2...gxh5 3.Bf2 with mate in 6.

  • 3. bxa3 b2 4. Rb1 gxh5 5. Rxh5 Qb3 6. Rh7 Qc2 7. Rg7+ Kh8 8. Rh7 Qxh7** -+.

  • The sacrifice 3. Bxg6 fxg6 is harmless, Black can bring in defenders via the opened 7th rank.

  • After 3. Bg4 or 3. Bf3 or 3.Be2, it's a draw, e.g. 3. Bf3 Nc2 4. Bd2 a3 7. Rg1 axb2 and White is one move short. 8. Rg4 b1=Q+ 2. Kg2 Ne1+ loses, the queen either goes to e1 or to f5 and takes one rook.

  • However, White seems to actually gain a small advantage by 3.Bd2 Nb5 4.Rg1 Qa7 5.Rg4 Nd6 6.Nxf7 Qxf7 7.Bxg6 Qxg6 8.Rxg6+ Kf7 9.Rg7+ Ke8 10.exd6 with +1.0 ~ +2.0.

  • Never mind, 3. Bf2 wins for White (because he can later shield his king by Bg1, allowing the king to stay out of reach of the Nc2 and thus gain that critical one tempo he needs to mate). Neat.

  • The same idea applies in the 1...Nxa3 variation: 2. Rxh7 Nc2 3. Bf2! a3 4. Rg1 axb2 5. Rg7+ Kh8 6. Rg3 b1=Q+ 7. Bg1 and Black can't stop the mate.

Well, it looks like the position is won for White after all, the engine just can't see it from the beginning.

Why it can't is a different question - that is already answered in length elsewhere on this site. In this instance, I guess the constant option of perpetual check combined with the serious material disadvantage completely throws off the pruning mechanism.

  • Trust me, I've analysed deeply but stockfish is clueless at depths 43. h5 and h6 are lost for black. For example, 1. Ng5 h5 2. Bxh5 Nxa3 3. Bd2 Nb4 4. Rg1, and there's no way for black to stop the plan of Rg4, Rgh4, and mating without giving up too much. It seems that if black responds with 3. Ne3, the knight can just be captured and the previous plans apply. Nxa3 is the only move I don't understand, if it holds to a draw or not. What's the concept behind it if white doesn't take it? – edit_profile Jan 26 at 2:35
  • @edit_profile At a very basic level, 1...Nxa3 kinda saves Black one tempo compared to the 1...h5 variations. – Annatar Jan 26 at 7:23
  • Why does it get played at all? To open the a-file so that Black can push pawns. – Annatar Jan 26 at 7:27
  • I believe that pushing the pawns will not save black. There must be some other motive with Nxa3 that I can't understand? – edit_profile Jan 26 at 22:13
  • @edit_profile The motive is to distract one rook so that White cannot mate. – Annatar Jan 27 at 7:47

Who concluded that this was a draw? Was there a triple repetition of position? Also there is no immediate forced checkmate after Ng5. Black has a huge material advantage (34 to 26 by point count) and can return a lot of material to try to avoid the type of checkmate you envision, e.g. by using the knight to distract White on the queenside, say by ..., Nxa3 to attempt to free up the Black "b" pawn. But as you indicate, White doesn't necessarily have to react to these distractions. But then Black can bring the knight back toward the kingside via e3 and f5 with interim threats, e.g. on the QR, to impede White's plan. That also frees up Black's "a" pawn to advance. I've played out the position several times, and there are too many lines to elaborate on, but giving back whatever material is necessary to avoid a checkmate, even still retaining a lead, may ultimately save Black. I prefer White's position though.

  • I've checked upon many lines but black has to return a queen and a knight at the minimum. After that the f7 pawn is weak which white can capture and create a passed pawn. So I don't giving back material is the answer. – edit_profile Jan 24 at 22:17
  • I'm agreeing with you that it's more than likely a win for White. Black has let his pieces get trapped on the Queenside. His only hope is probably a blunder by White. – CConero Jan 24 at 22:25

The key defensive idea to me seems either breaking through on a3/c3/b2 or sacrificing the knight on e5 and then taking the pawn on f6 or (after dxe5) playing d4 exposing the white king on h1 to all kinds of nastiness. So after 1.Ng5 h5 2.Bf2 Nb4 and now after 3.Bh5 Nd3 Black is ready for plan #2. After 3.ab4 a3 4.ba3 b2 Black continues with Qa7 / Rb7 to laterally defend on g7/h7 against sacrifices on e6 / g6. The computer thinks this is a draw as black cannot prevent the perpetual check with a rook on h7/g7. Actually, the position is totally chaotic and I would certainly prefer the white pieces against any human.

  • Well I analyzed h5 and it is a win for white, you can break it open with Bxh5 and the relentless aggression works out. I think Nxa3 might be the only reason this position could be a draw, and I don't see why – edit_profile Jan 25 at 14:06

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