#### Only Chess Forum

1.  moonbus
Uber-Nerd
21 Jan '17 19:11
mate in two. (White is moving up the board.) PM me your solution.

Hint:
Your first instinct will mate, but not in two. Think again, harder.
2.  krieghoff
Carrobie was here
22 Jan '17 03:24
1. b8=N : a6 (forced 2. Nxc6 ++
3.  moonbus
Uber-Nerd
22 Jan '17 08:13
Well done! Thank you for all of your replies; there were more than I could reply to personally due to a PM limit (spam filter!) at RHP. The First Responders were:

robbebop (1588)

vandervelde (2116)

thaughbaer (1779)

HikaruShindo (1661)

krieghoff (2421)

I suppose krieghoff and vandervelde saw this instantly. For the rest of us no-where-near-IM-level duffers, I offer the following explanation, what my thought process was in solving this.

The puzzle intrigued me because it is an example of Emanuel Lasker's remark that when you see a good move, wait, keep looking because you might find something better. My first instinct, and maybe yours too, was to promote the b-pawn to a queen. Black then plays Ka6 and will be mated, eventually, but not on the next move.

So, I had to think again, harder, and under-promote (an interesting topic in itself).

At this point, I stopped thinking about what White could move and started thinking about Black instead. The Black K is very nearly stalemated already: he has only one legal square left, a6. Take away a6, and any check will be checkmate. Under-promoting to a N on b8 would take away a6 from the Black K; now a quick look is needed to ensure that Black still has a legal move if a N is on b8 — we don't want to stalemate him. Yes, Black can still play a7-a6 — as krieghoff noted above, forced. This is good, because it traps the Black K, and now any check at all will be checkmate.

From here, it is a short hop to seeing that Nxc6 is mate.
4. 22 Jan '17 18:02
One interesting thing is that I had a different when solving the problem. I looked at promotion to a queen, and then checkmate after 1…a6. It was only as I checked that I realized Black had 1…Ka6. From there, though, I found the knight promotion almost immediately, and checked it several times before sending a pm (not wanting to be wrong.) I think it's a symptom of a larger tendency where I miss moves a lot (even OTB, when I take my time more.)
5. 23 Jan '17 13:51
In a game situation I would have gone with the queen just because it was safer without calculating. I don't trust my calculations without a constant check because I am hardly ever correct when trying to calculate.
6.  moonbus
Uber-Nerd
24 Jan '17 08:11
Good point. Calculation is error prone. GMs play more on general principles than on long I-go-here-you-go-there-I-go-here-you-go-there calculations. How not to calculate would be a good topic for a blog.