Answers.Body: <p>Addendum 6/24/2023: It turns out that <strong>20 pieces</strong> actually <em>can</em> be done in a manner that feels so obvious now. Removing wPf6 would cause an illegal position, and removing wBg7 would leave f6 unguarded.</p>
  <pre><code> [FEN &quot;8/2B1R1B1/1N1rnP2/1Qq1knR1/2pP1r2/2K1bPB1/4R3/8 w - - 0 1&quot;]
  </code></pre>
  <hr />
  <p>I found another, far funnier, reason why a piece can't be removed; the position would otherwise be illegal!</p>
  <p>Using this idea, I've managed 19 pieces. White's pieces can't be taken, with the White king unmoveable, or else the checkmate is null. Black's pieces prevent illegality and &quot;guard&quot; c4, minus the e6 block.</p>
  <pre><code> [FEN &quot;8/2B1R3/1N1rnp2/1Qq1knR1/2pP1r2/2K1bPB1/4R3/8 w - - 0 1&quot;]
  </code></pre>
  
Answers.ContentLicense: CC BY-SA 4.0
Answers.CreationDate: 2024-02-18T11:21:34.780Z
Answers.Text: In the first position the c4 pawn unnecessary.
Answers.UserDisplayName: blademan9999
Answers.ContentLicense: CC BY-SA 4.0
Answers.CreationDate: 2024-02-18T12:29:53.653Z
Answers.Text: @blademan9999 You are absolutely incorrect.  It is needed for the wK to not be in check.
Answers.UserDisplayName: Rewan Demontay
Answers.ContentLicense: CC BY-SA 4.0
Answers.CreationDate: 2024-02-27T03:27:15.633Z
Answers.Text: In check by what, exactly? It's inbetween the two kings.
Answers.UserDisplayName: blademan9999
Answers.ContentLicense: CC BY-SA 4.0
Answers.CreationDate: 2024-02-28T00:46:45.407Z
Answers.Text: @blademan9999 What are you on about? If the bPc4 is not there, the position is illegal from the bQ checking the wK.
Answers.UserDisplayName: Rewan Demontay
Answers.ContentLicense: CC BY-SA 4.0
Answers.CreationDate: 2024-03-10T15:46:48.500Z
Answers.Text: So, I got the pawns mixed up.
Answers.UserDisplayName: blademan9999
Answers.ContentLicense: CC BY-SA 4.0
Answers.CreationDate: 2021-03-08T17:32:59.763Z
Answers.LastActivityDate: 2024-02-18T12:31:32.070Z
Answers.LastEditDate: 2024-02-18T12:31:32.070Z
Answers.UserDisplayName: Rewan Demontay
Answers.Body: <p>I hope I understood it right...</p>
  <pre><code>[FEN &quot;1B2R2B/1Nq1b3/2P2n2/4kP2/2K2n2/4bP2/Qr5B/B3R3 w - - 0 1&quot;]
  </code></pre>
  <p>Also 19, but more chances to improve it. (The problem is that the
  king should not be superfluous. If the question had been formulated
  as &quot;#, -any piece, not #, maybe because illegal&quot;, I get 20.)</p>
  
Answers.ContentLicense: CC BY-SA 4.0
Answers.CreationDate: 2021-03-09T09:39:00.260Z
Answers.LastActivityDate: 2021-06-17T19:57:30.043Z
Answers.LastEditDate: 2021-06-17T19:57:30.043Z
Answers.UserDisplayName: Hauke Reddmann
Body: <p>What is the maximum number of black and white pieces which are relevant in a checkmate position (by White)? That is if any piece (black or white) was removed from the board, the checkmate would not be possible because that piece is relevant or has a role in such checkmate position because it may be: (1) checking the king (2) blocking a square where the checked king can move (3) attacking the unoccupied square where the checked king can move (4) preventing the king or other pieces to capture or block the checking piece. (Note that there shall be no idle or useless piece including the White king)</p>
  
Tags: <problems><checkmate>
Title: Checkmate with only relevant pieces
ThreadText: # Checkmate with only relevant pieces
  
  (Asked by: username_0 on 2021-03-08)
  
  What is the maximum number of black and white pieces which are relevant in a checkmate position (by White)? That is if any piece (black or white) was removed from the board, the checkmate would not be possible because that piece is relevant or has a role in such checkmate position because it may be: (1) checking the king (2) blocking a square where the checked king can move (3) attacking the unoccupied square where the checked king can move (4) preventing the king or other pieces to capture or block the checking piece. (Note that there shall be no idle or useless piece including the White king)
  
  **Tags:** problems, checkmate
  
  
  ---
  
  ## 2 answers
  
  ### Answer by username_1 on 2021-03-08. Score: 5 | Accepted answer
  
  Addendum 6/24/2023: It turns out that **20 pieces** actually *can* be done in a manner that feels so obvious now. Removing wPf6 would cause an illegal position, and removing wBg7 would leave f6 unguarded.
  
  ```
   [FEN "8/2B1R1B1/1N1rnP2/1Qq1knR1/2pP1r2/2K1bPB1/4R3/8 w - - 0 1"]
  
  ```
  
  ---
  
  I found another, far funnier, reason why a piece can't be removed; the position would otherwise be illegal!
  
  Using this idea, I've managed 19 pieces. White's pieces can't be taken, with the White king unmoveable, or else the checkmate is null. Black's pieces prevent illegality and "guard" c4, minus the e6 block.
  
  ```
   [FEN "8/2B1R3/1N1rnp2/1Qq1knR1/2pP1r2/2K1bPB1/4R3/8 w - - 0 1"]
  
  ```
  
  > **Comments:**
  > **username_2**: In the first position the c4 pawn unnecessary.  
  > **username_1**: @username_2 You are absolutely incorrect.  It is needed for the wK to not be in check.  
  > **username_2**: In check by what, exactly? It's inbetween the two kings.  
  > **username_1**: @username_2 What are you on about? If the bPc4 is not there, the position is illegal from the bQ checking the wK.  
  > **username_2**: So, I got the pawns mixed up.  
  
  ---
  
  ### Answer by username_3 on 2021-03-09. Score: 2
  
  I hope I understood it right...
  
  ```
  [FEN "1B2R2B/1Nq1b3/2P2n2/4kP2/2K2n2/4bP2/Qr5B/B3R3 w - - 0 1"]
  
  ```
  
  Also 19, but more chances to improve it. (The problem is that the
  king should not be superfluous. If the question had been formulated
  as "#, -any piece, not #, maybe because illegal", I get 20.)
  
  
  ---
  
  