- OpeningBlackmar–Diemer Gambit
- The mistake7…Bg4
- The endCheckmate
The trap
The Blackmar–Diemer Gambit arises after 1.d4 d5 2.e4 dxe4 3.Nc3 Nf6 4.f3. White gives up a pawn, often two, to get open lines against Black's king.
After 4…exf3 5.Qxf3 the pawn on d4 is unprotected. Black takes it, and after 6.Be3 Qb4 7.O-O-O White has castled long with the rook on the open d-file.
1. d4 d5 2. e4 dxe4 3. Nc3 Nf6 4. f3 exf3 5. Qxf3 Qxd4 6. Be3 Qb4 7. O-O-O Bg4 8. Nb5 Bxf3 9. Nxc7#
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Why it's easy to fall for
7…Bg4 is a natural move. The bishop comes out and attacks White's queen at the same time. Most people don't look further than that: the queen has to move, and Black has gained a tempo.
But Black's king is still on e8, and the d-file is open. The black pawn on c7 is guarded only by the king. After 8.Nb5! White threatens 9.Nxc7 mate, and that threat is bigger than Black's threat against the queen.
What Black should have played
Play 7…c6. The pawn takes the b5 square away from the knight, and Black keeps both extra pawns. The position is good for Black.
7… c6
Right. The pawn takes the b5 square away from the knight. Black keeps both extra pawns and stands well.
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How it ends
If Black takes the queen, it's mate on move nine. If Black doesn't take it, for example with 8…Na6, which guards c7, White is still winning: White's queen gets away, and Black's position falls apart. A handbook from 2000 gives White only some advantage after 9.Qxb7 Qe4 10.Qxa6 Qxe3+, but White's edge is bigger than that.
How often does it happen?
Our numbers reach as far as 5…Qxd4, which 124,170 of the 976,818 games between club players in that position played (13%). White won 53% of those games. After that the position is too rare to be among the positions we have counted, so we can't say how often the mistake itself is made.
Our numbers reach as far as 5…Qxd4, which 14,547 of the 112,462 games between strong players in that position played (13%). White won 48% of those games. After that the position is too rare to be among the positions we have counted, so we can't say how often the mistake itself is made.
Move by move: club players
| Move | Games in the position | Played the move | Then White won | Draw | Black won |
|---|---|---|---|---|---|
| 5.Qxf3 | 2,364,488 | 909,023 (38%) | 51% | 3% | 45% |
| 5…Qxd4 | 976,818 | 124,170 (13%) | 53% | 3% | 44% |
Move by move: strong players
| Move | Games in the position | Played the move | Then White won | Draw | Black won |
|---|---|---|---|---|---|
| 5.Qxf3 | 541,726 | 106,207 (20%) | 47% | 4% | 49% |
| 5…Qxd4 | 112,462 | 14,547 (13%) | 48% | 3% | 49% |
"Played the move": of the games in the position, how many continued with the move of the trap line; then how those games ended. A position missing from the table is too rare for our numbers. Source: games on Lichess in blitz, rapid and classical, fetched 6 and 7 November 2026. Club players means games where the players average 1600 or more, strong players games where they average 2000 or more.
Explore the position on the analysis board: the moves club players choose here and how the games tend to end.
Try it
Black has just attacked the queen with 7…Bg4. White to move: leave the queen and find the mate!
Your opponent moves…
The history
The trap is named after Hermann Halosar. He had Black against Emil Josef Diemer, the gambit's great advocate, in Baden-Baden in 1934 and lost in this line. He didn't fall for the mate himself: he guarded c7 with 8…Na6, and the game went on 9.Qxb7 Rc8 10.Qxa6.
The gambit has two names. Armand Blackmar of New Orleans made 1.d4 d5 2.e4 dxe4 3.f3 known in the late 19th century. Diemer instead made 3.Nc3 with 4.f3 popular in the 20th century.
Frequently asked questions
What is the Halosar Trap?
A trap in the Blackmar–Diemer Gambit. Black attacks White's queen with 7…Bg4, White replies 8.Nb5! and threatens mate on c7. If Black takes the queen, 9.Nxc7 is mate.
How does Black avoid the Halosar Trap?
By playing 7…c6 instead of 7…Bg4. The pawn takes b5 away from the knight, and Black keeps both extra pawns.
Every move on this page has been checked against the rules of chess, and every judgement of who stands better has been checked with a chess computer.
Updated November 14, 2026
7. O-O-O
White castles long. The king guards b2, and the rook takes the d-file.