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Admirers of retrograde analysis may like this puzzle:

From the initial position (not Fischerandom) White moves, Black moves, White moves, Black moves, and now White can checkmate Black with 20 different legal moves. 6 legal moves will not checkmate from that position. Can you find the moves?

Just a note, 1.e4 f6 2.d4 g5 3.Qh5# is not a solution, because only the move 3.Qh5# will checkmate, not any other legal move.

Jesper Norgaard wrote: ↑

Sat Apr 14, 2018 10:08 am

Admirers of retrograde analysis may like this puzzle:

From the initial position (not Fischerandom) White moves, Black moves, White moves, Black moves, and now White can checkmate Black with 20 different legal moves. 6 legal moves will not checkmate from that position. Can you find the moves?

Just a note, 1.e4 f6 2.d4 g5 3.Qh5# is not a solution, because only the move 3.Qh5# will checkmate, not any other legal move.

When you say "checkmate Black with 20 different legal moves", do you mean checkmate at some future point in the game, or actually deliver checkmate with the 3rd move of the game?

The latter. There is a genuine solution (e.g. 2 moves from White and Black) and then there is 20 different checkmate moves possible as the 3rd move of the game. I am validating with the FIDE Laws of Chess, which should be a hint. Perhaps the hustlers in New York Central Park would not accept it, but they usually don't follow FIDE Laws of Chess anyhow.

Is it something absurd like 1.Ra1c5 h7d3 2.Qe5 Kd4? Where the checkmate is legal provided the move delivering checkmate is legal, regardless of what illegalities preceded it?

FIDE loves absurd checkmates, and will go to their grave defending them. I think these checkmates should be abolished.

I did state that the last move needs to be legal, however I did not say that about the preceding moves, and as such I will accept your solution. However, let me rephrase the puzzle: 4 half moves are made from the initial position, from which only 1 is illegal. Can you find the moves, producing a position from which 20 legal checkmates are possible, and 6 legal moves are not checkmates? You are close, except all 4 of your half moves are illegal.

I'm not sure that I've understood the 'rules' but how about 1.c4 e6 2. e3 Kc2. Assuming that 3.Qxc2 is illegal because you can't take a king, then there appear to be six moves (Qe2, Qf3, Qg4, Qh5, Bd3, Ne2) which don't result in checkmate and another twenty which do.

Kevin Williamson wrote: ↑

Sat Apr 14, 2018 9:11 pm

I'm not sure that I've understood the 'rules' but how about 1.c4 e6 2. e3 Kc2. Assuming that 3.Qxc2 is illegal because you can't take a king, then there appear to be six moves (Qe2, Qf3, Qg4, Qh5, Bd3, Ne2) which don't result in checkmate and another twenty which do.

Why not go 1.a4 g5 2.h4 Kxc2 and give White some extra checkmating moves.

Kevin Williamson wrote: ↑

Sat Apr 14, 2018 9:11 pm

I'm not sure that I've understood the 'rules' but how about 1.c4 e6 2. e3 Kc2. Assuming that 3.Qxc2 is illegal because you can't take a king, then there appear to be six moves (Qe2, Qf3, Qg4, Qh5, Bd3, Ne2) which don't result in checkmate and another twenty which do.

I get 19 moves that are checkmate:
a3 a4 b3 b4 c5 Na3 Nc3 d3 d4 e4 Be2 Nf3 Nh3 f3 f4 g3 g4 h3 h4

Chris Goodall wrote: ↑

Sat Apr 14, 2018 10:37 pm

Why not go 1.a4 g5 2.h4 Kxc2 and give White some extra checkmating moves.

I get also 19 moves that are checkmate:

a5 b3 b4 d3 d4 e3 e4 f3 f4 g3 g4 hxg5 h5 Na3 Nc3 Nf3 Nh3 Rh2 Rh3

A big problem is that Ra2 and Ra3 are not checkmate since Black can play Kxb1

My intended solution was 1.d4 f5 (a Dutch Defence!) 2.Nc3 Ke4
19 checkmate moves:
a3 a4 b3 b4 e3 f3 f4 g3 g4 h3 h4 Nf3 Nh3 Qd2 Qd3 Be3 Bg5 Bh6 Rb1

Actually there are 8 moves that are not checkmate (I forgot Kd2 and Bd2):
Nd5 Nb5 Na4 Nb1 Bf4 d5 Kd2 Bd2

1.c4 e6 2.Nf3 Kc2

There are 21 checkmate moves:
a3 a4 b3 b4 c5 d3 d4 e3 e4 g3 g4 h3 h4 Nd4 Ne5 Ng5 Nh4 Ng1 Rg1 Na3 Nc3

However, astonishingly there are no legal moves that are not checkmate!

Jesper Norgaard wrote: ↑

Sun Apr 15, 2018 12:51 am

Kevin Williamson wrote: ↑

Sat Apr 14, 2018 9:11 pm

I'm not sure that I've understood the 'rules' but how about 1.c4 e6 2. e3 Kc2. Assuming that 3.Qxc2 is illegal because you can't take a king, then there appear to be six moves (Qe2, Qf3, Qg4, Qh5, Bd3, Ne2) which don't result in checkmate and another twenty which do.

I get 19 moves that are checkmate:
a3 a4 b3 b4 c5 Na3 Nc3 d3 d4 e4 Be2 Nf3 Nh3 f3 f4 g3 g4 h3 h4

Don't forget 3.Ke2, so I think it is 20.

I don't get this. What's the point of a Dutch defence where the second move is illegal??

Nick Ivell wrote: ↑

Sun Apr 15, 2018 10:36 am

I don't get this. What's the point of a Dutch defence where the second move is illegal??

I'd be inclined to ask what's the point of a Dutch Defence where the 2nd move is legal

Jesper Norgaard wrote: ↑

Sun Apr 15, 2018 12:51 am

Kevin Williamson wrote: ↑

Sat Apr 14, 2018 9:11 pm

I'm not sure that I've understood the 'rules' but how about 1.c4 e6 2. e3 Kc2. Assuming that 3.Qxc2 is illegal because you can't take a king, then there appear to be six moves (Qe2, Qf3, Qg4, Qh5, Bd3, Ne2) which don't result in checkmate and another twenty which do.

I get 19 moves that are checkmate:
a3 a4 b3 b4 c5 Na3 Nc3 d3 d4 e4 Be2 Nf3 Nh3 f3 f4 g3 g4 h3 h4

Ke2 is surely the 20th

[Edit: apologies hadn't seen that this correction had already been made]