Two Simple Xmas Puzzles

Reminder: You are invited to solve the 2 Christmas puzzles in post #1. There are no prizes but I think you will enjoy them when you have some knowledge of retrograde problems - puzzles which require you to look back into the past of a chess diagram.

You may need to read up on the current basic chess rules and here is the link: Chess Handbook, Article 1 through 5.

I will post the solutions in about 3 days!

Solution to problem A

Solution to problem B (the starting position is obviously legal)

Thanks, Colin20G!

Solution A is great, B falls short! Look at the answers I gave to similar solutions on the first page. Check out the Chess Handbook link to find the chess rule you overlooked.

B) solution

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-1...Kh8xNg8 -2.Nh6xRg8 & 1.Nf7#

The trouble with uncapturing a white Queen is "Dead Reckoning" - per rule, the game is immediately drawn after white plays Qg8+, since there is no non-drawing legal continuation possible at that point. Black cannot play KxQ, so neither can he retract that move from the starting diagram.

In my solution, after Black retracts -1...Kh8xNg8 he can forward-play 1...Bxg6 and it is possible for either side to get checkmated still.

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Nice problem!

Edit: The FIDE rule in question is 5.2.2.

Simple puzzles  Yeah right.   Retrograde eludes me I am ignoring posted answers and still trying though.

Well done, BigDoggProblem! You know the rules and you know how to apply them!

That a pretty silly rule, but it makes retro puzzles a lot more interesting.

That's a good observation! A lot more can said about this rule which also had an effect on other problem compositions. Since 2015, it no longer applies to common problem types and endgame studies but remains in force for chess games and problems which are evidently retrograde.

Very interesting! Had a hard thought espacially about the second one once I learned about the rule.

But I think I have it:

The black king retracts from g8 to h8 (uncapturing a Knight). The uncaptured Knight retracts from g8 to h6 (uncapturing a rook). We reach the position where white could have mated and that would be with Nf7#.  

Great puzzles. 

You're spot on! It's a nice way to learn about the rules of chess. In a few days I'll post the solutions and give some details on the alternatives.

Problem B is tricky...

Last reminder: Thanks for all the reactions sofar! There are still 1 or 2 days before I post the official solution.

If you like this type of problem here's an oldie from Xmas 2016: https://www.chess.com/forum/view/more-puzzles/take-back-your-quotsorry-movequot---for-once#comment-31186188. You can find the solution in that thread!

And Rocky64 enjoys to receive a lot of solutions to his current Christmas problem-solving competition. He has some nice prizes for those who solve any of his 3 puzzles!

Chess composed puzzles are a beautiful topic. Thanks for sharing!

The two simple solutions

Solution Puzzle A.

Below the diagram of the position before the correct last move ... Kg8xBh8. In this diagram black can play either .... Kxh8 which gets us in the puzzle position or .... Kf8 and be checkmated by Bg7# which is the solution checkmate.

What happens if black tries to retract a different legal move from the puzzle diagram?

A. ... Bg8xPh7. Now with any luck white could checkmate black on his next move by Ph7xBg8=Q#. But he is out of luck because black must first play something to replace the move he has just taken back. Every black play though destroys the white checkmate plan.

B. ... KxNh8 almost works but after black replaces it with Kf8, the intended Ng6+ fails to .... Bxg6.

C. ... Bb1xQh7 brings too much power on the board. With his only legal replacement move ... Kxh7 black must recapture the white queen and the checkmate hopes are gone

Solution Puzzle B.

This puzzle is not just about "retracting" moves but also about a special rule in chess. It's article 5.2.2. in the FIDE laws which replaces older competition rules regarding "insufficient material" - which no longer exist (except in some competitions).

5.2.2 The game is drawn when a position has arisen in which neither player can checkmate the opponent’s king with any series of legal moves. The game is said to end in a ‘dead position’. This immediately ends the game .......

The logical point to grasp here is that you cannot continue a game beyond the point of where it is dead. Even when players fail to notice that event and continue playing, article 5.2.2 says that "eating a snack" is as much part of the game as the moves following a dead posiiton! So, not at all.

The corollary for retraction is that you cannot retract moves into a dead position. If the position preceding a diagram were dead, then the game would have ended there and could not be legally extended to the diagram. But you might say that a puzzle is not a game. That's a fair opinion but then of course you shouldn't have tried to solve a problem of the retrograde type which totally depends on analyzing (imaginary) game histories. On top of that, the composition rules prescribe that all (standard) composition types - like endgame studies and checkmate problems - may only use diagrams for which a legal (proof) game exists. "Funny" diagrams are not allowed!

What does it all mean for puzzle B? Diagram:

Note that the diagram position of puzzle B is dead. No side could ever checkmate the other side even with its assistance. Which means that you must be careful when taking back moves. For instance, the last move ...Kh8-g8 would be illegal because the position with the king on h8 is dead as well! You can only take back a move into a position where checkmate is still possible. That makes the following last moves illegal: ...Kh8-g8, ....Kf8-g8, ....Kh8xQ8, ...Kf8xQg8, ...Kh8xRg8, ...Kf8xRg8, ...Kh8xBg8...Kf8xBg8. You may wonder why the rook- and queen captures are illegal. The answer is that - though sufficient mating material is on the board - black is forced to capture that check-giving intruder on the next move! And then the mating potential is out the window again. Therefore, dead. The only remaining and legal retractions are ......Kh8xNg8 and ...Kf8xNg8. But won't the knight be eliminated on the next move as well? No it does not because it doesn't check the black king. Instead black could have played Bh7xg6!! leaving mating potential on the board. Example diagram:

The rest of the solution is simple when you solved puzzle A or have some experience with retraction puzzles. White needs to retract after black and then replace his retracted move with a checkmate move. The idea of the puzzle instructions was to communicate to you that black and white cooperate in their retractions. So black retracts a move which helps white to later checkmate him. That only happens when black retracts ...Kh8xNg8! Subsequently white retracts Nh6xRg8! and checkmates instead with Nf7#. Capisce?

Diagram after the retractions: