Answers.Body: <p>On the question about your opponent: maybe it's notable that you and they both played 18 moves down this theoretical line. But that's just as much on you as on them. After that, their 19th and 20th moves are OK but both outside the top 5 engine choices - then you threw away a piece on move 20 and after that their moves are basically obvious and pretty reasonable at a 1600 level. On move 23 the engine suggests that they had several stronger continuations available (although their 23. Rg5 is nonetheless winning).</p>
  <p>For the rest: perhaps someone else can give a good strategic comparison of the different plans for Black, but at a superficial level, your question rather answers itself? The algorithm reports 17...c5 as an inaccuracy because it evaluates the position after 17...c5 as +0.81, whereas it evaluates the position after 17...Nb6 as +0.32. Quite possibly it has not devoted much compute time to this evaluation, and it might change its mind with deeper consideration.</p>
  <p>It reports 17. a4 as the last book move because 17. a4 is in its book while 17... c5 isn't. Someone else may be able to answer with more information about the chess.com &quot;book&quot;, but 17 moves is already quite deep and it doesn't seem unreasonable for an opening book to run out somewhere around there? (Or maybe the book doesn't run out, but 17... Nb6 is in the book instead of 17...c5 because it's genuinely regarded as the superior move?)</p>
  
Answers.ContentLicense: CC BY-SA 4.0
Answers.CreationDate: 2024-12-30T12:45:05.800Z
Answers.LastActivityDate: 2024-12-30T12:45:05.800Z
Answers.UserDisplayName: James Martin
Body: <pre><code>[FEN &quot;&quot;]
  [StartFlipped &quot;1&quot;]
  [StartPly &quot;34&quot;]
  
  1. e4 e5 2. Nf3 Nc6 3. Bb5 a6 4. Ba4 Nf6 5. O-O Be7 6. Re1 b5 7. Bb3 d6 8. c3 O-O 9. h3 Nb8 10. d4 Nbd7 11. Nbd2 Bb7 12. Bc2 Re8 13. Nf1 Bf8 14. Ng3 g6 15. Bg5 h6 16. Bd2 Bg7 17. a4 c5 18. d5 c4 19. Qc1 Kh7 20. Nh4 Nxe4 21. Rxe4 Nc5 22. Rg4 h5 23. Rg5 Bh6 24. Rxh5 1-0
  </code></pre>
  <p>I played the game above with black and here is part of the game review by chess.com:</p>
  <p><a href="https://i.sstatic.net/0za4JrCY.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/0za4JrCY.png" alt="enter image description here" /></a></p>
  <p><a href="https://i.sstatic.net/6HxPIgYB.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/6HxPIgYB.png" alt="enter image description here" /></a></p>
  <p>Chess.com tells me that</p>
  <ol>
  <li>
  <ol start="17">
  <li>a4 is the last book move;</li>
  </ol>
  </li>
  <li>
  <ol start="17">
  <li>... c5 is an inaccuracy.</li>
  </ol>
  </li>
  </ol>
  <p>However, based on the lichess.org database (see below), 17. ... c5 has been played over 100 times by masters including world champions such as Carlsen and Spassky. Furthermore, 17. ... c5 is the most popular move in this position. My questions:</p>
  <ol>
  <li>Why doesn't chess.com consider 17. ... c5 a &quot;book move&quot;?</li>
  <li>Why does chess.com consider 17. ... c5 as an inaccuracy?</li>
  </ol>
  <p>Just a side note: My opponent is rated 1600+ and the time control of this game is 3+2. Chess.com gave his/her game rating as 2350 with accuracy 95.4%. I am not sure if he/she used external help.</p>
  <p><a href="https://i.sstatic.net/LRpXRBNd.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/LRpXRBNd.png" alt="enter image description here" /></a></p>
  
Comments.ContentLicense: CC BY-SA 4.0
Comments.CreationDate: 2024-12-30T12:00:39.373Z
Comments.Text: https://chess.stackexchange.com/questions/39880/what-does-it-mean-when-stockfish-evaluates-a-move-as-an-inaccuracy-after-previou Closely related, maybe duplicate.
Comments.UserDisplayName: Allure
Tags: <opening><engines><online-chess><ruy-lopez>
Title: Why doesn't chess.com like my 17... c5?
ThreadText: # Why doesn't chess.com like my 17... c5?
  
  (Asked by: username_0 on 2024-12-30)
  
  ```
  [FEN ""]
  [StartFlipped "1"]
  [StartPly "34"]
  
  1. e4 e5 2. Nf3 Nc6 3. Bb5 a6 4. Ba4 Nf6 5. O-O Be7 6. Re1 b5 7. Bb3 d6 8. c3 O-O 9. h3 Nb8 10. d4 Nbd7 11. Nbd2 Bb7 12. Bc2 Re8 13. Nf1 Bf8 14. Ng3 g6 15. Bg5 h6 16. Bd2 Bg7 17. a4 c5 18. d5 c4 19. Qc1 Kh7 20. Nh4 Nxe4 21. Rxe4 Nc5 22. Rg4 h5 23. Rg5 Bh6 24. Rxh5 1-0
  
  ```
  
  I played the game above with black and here is part of the game review by chess.com:
  
  [![enter image description here](https://i.sstatic.net/0za4JrCY.png)](https://i.sstatic.net/0za4JrCY.png)
  
  [![enter image description here](https://i.sstatic.net/6HxPIgYB.png)](https://i.sstatic.net/6HxPIgYB.png)
  
  Chess.com tells me that
  
  1. 17. a4 is the last book move;
  2. 17. ... c5 is an inaccuracy.
  
  However, based on the lichess.org database (see below), 17. ... c5 has been played over 100 times by masters including world champions such as Carlsen and Spassky. Furthermore, 17. ... c5 is the most popular move in this position. My questions:
  
  1. Why doesn't chess.com consider 17. ... c5 a "book move"?
  2. Why does chess.com consider 17. ... c5 as an inaccuracy?
  
  Just a side note: My opponent is rated 1600+ and the time control of this game is 3+2. Chess.com gave his/her game rating as 2350 with accuracy 95.4%. I am not sure if he/she used external help.
  
  [![enter image description here](https://i.sstatic.net/LRpXRBNd.png)](https://i.sstatic.net/LRpXRBNd.png)
  
  **Tags:** opening, engines, online-chess, ruy-lopez
  
  > **Comments:**
  > **username_1**: https://chess.stackexchange.com/questions/39880/what-does-it-mean-when-stockfish-evaluates-a-move-as-an-inaccuracy-after-previou Closely related, maybe duplicate.  
  
  ---
  
  ## 1 answers
  
  ### Answer by username_2 on 2024-12-30. Score: 5
  
  On the question about your opponent: maybe it's notable that you and they both played 18 moves down this theoretical line. But that's just as much on you as on them. After that, their 19th and 20th moves are OK but both outside the top 5 engine choices - then you threw away a piece on move 20 and after that their moves are basically obvious and pretty reasonable at a 1600 level. On move 23 the engine suggests that they had several stronger continuations available (although their 23. Rg5 is nonetheless winning).
  
  For the rest: perhaps someone else can give a good strategic comparison of the different plans for Black, but at a superficial level, your question rather answers itself? The algorithm reports 17...c5 as an inaccuracy because it evaluates the position after 17...c5 as +0.81, whereas it evaluates the position after 17...Nb6 as +0.32. Quite possibly it has not devoted much compute time to this evaluation, and it might change its mind with deeper consideration.
  
  It reports 17. a4 as the last book move because 17. a4 is in its book while 17... c5 isn't. Someone else may be able to answer with more information about the chess.com "book", but 17 moves is already quite deep and it doesn't seem unreasonable for an opening book to run out somewhere around there? (Or maybe the book doesn't run out, but 17... Nb6 is in the book instead of 17...c5 because it's genuinely regarded as the superior move?)
  
  
  ---
  
  