Pending moment · story 1b858313-6a51-4eed-b45e-3d7e5ea90441 · room 4ac1aed1 · move 31 · reason positional-gives-material · 2026-09-04

Case 1b858313 move 31: the queen steps onto the diagonal where the next check also hits it

1. The position before the move

White to move. FEN 8/6pk/5p1p/3Q4/8/7P/6PK/4q3 w - - 8 45

White: king h2, queen d5, pawns g2 and h3. Black: king h7, queen e1, pawns f6, g7, h6. Black has one pawn more. The engine says the game is level (-21 cp for White with the best move) because the queens are on the board and the black king is open.

The one thing to see: the diagonal h2 to e5. The white king sits at the end of it. Any black queen check on e5 also looks along the rank e5 to d5 and along the diagonal e5 to d4. So a white queen on d4 or d5 can be hit by that check at the same time as the king.

2. What the played move does: Qd4

1. Qd4. The queen goes to d4, on the e5 to h2 diagonal.

1... Qe5+. Check to the king on h2, and the same queen attacks the queen on d4.

2. Qxe5 fxe5. The queens are gone. Pawn ending, White two pawns against three.

After Qe5+ the white king must answer the check. The king moves lose the queen: Kh1 then Qxd4; Kg1 then Qxd4+. So White must trade queens with Qxe5, and fxe5 takes back. The pawn ending is lost: two pawns against three, and the black king is closer to the centre. Engine after Qd4: -541 cp for White. Before Qd4: -8 cp.

Engine line after Qd4: Qd4 Qe5+ Qxe5 fxe5 Kg3 Kg6 Kg4 h5+ Kf3 Kf5 g3 e4+ ...

3. What the better move does: Qc4

1. Qc4. The queen leaves the diagonal and the fifth rank.

1... Qe5+. Still check, but this time the check hits only the king.

2. Kg1. The king steps away. Both queens stay. The game stays level.

Qc4 does one thing: it keeps the queen off every line the check on e5 covers. After Qe5+ the king steps to g1 and nothing hangs. The black king on h7 is open, so White keeps checking chances and the extra black pawn does not count much. Engine line after Qc4: Qc4 Qe5+ Kg1 f5 Kh1 Kg6 Qc1 Kf6 ... at -21 cp.

4. What the story says today

This is where you moved your queen (Qd4). The idea makes sense, but there's a reply that causes real trouble. Let's look at it.
Your queen moves (Qd4).
You don't lose a piece right away, but after their reply (Qe5+), attacking your queen, they end up with the better position. Along the way the queens get traded.
Their queen moves (Qe5+). Your queen captures their queen (Qxe5). Their pawn captures your queen (fxe5).
The stronger choice was (Qc4). This keeps the game even instead of letting them take over. This move is easy to miss — few players find it.
Your queen moves (Qc4). Their queen moves (Qe5+). Your king moves (Kg1).

The story names the reply (Qe5+), says the queens get traded, and says the result is worse. It does not say why the trade is forced (the check hits king and queen at once) or why the ending is lost (a pawn down).

5. What our offline rules and the DecodeChess-style facts say

source	output
positional rules	`piece-hanging` at ply 2: qd4 after Qe5+ `piece-trapped` at ply 2: qd4 after Qe5+ `forced-queen-trade` at ply 4: Qd4 Qe5+ Qxe5 fxe5
DecodeChess-style facts, better move Qc4	intends Kg1 or Kh1 (no board fact)
DecodeChess-style facts, played move Qd4	intends Qxe5 or Kg3 (no board fact)

The one-ply facts see nothing, because Qd4 hangs nothing and threatens nothing by itself. The forced-queen-trade rule at ply 4 is the one that names the point. In DecodeChess terms this is "avoids an exchange" (E28) for Qc4 and "allows a check that forks king and queen" for Qd4. The sentence we should make: "Qd4 puts the queen on the diagonal of the coming check. Qe5+ hits king and queen at once, so the queens must come off, and the pawn ending is lost a pawn down. Qc4 keeps the queen off that line."

FEN to paste into DecodeChess: 8/6pk/5p1p/3Q4/8/7P/6PK/4q3 w - - 8 45