# Checkmate with only relevant pieces

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)

• Your stated requirement is only that a single piece cannot be removed without losing the mate (or legality). How about removing multiple units simultaneously? And do you agree with Rewan's idea illegality is another reason why a piece can't be removed? – Laska Mar 9 at 0:34
• ..a piece that prevent kings in check is also relevant for legality but not stated in OP. Rewan Demontay's 18 pieces earlier solution has some pieces preventing White king in check but they also block the king's escape. – TSLF Mar 9 at 17:34

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"]


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.)