Underpromotion in the key

We had referenced underpromotion when we had talk about Characteristic-9 of the key. The underpromotion is an element of surprise, only for those who have never occupied themselves with chess problems, whilst the problem solvers always have it under consideration.

	(Problem 30) Magazine “Lexorama #5” White plays and mates in 3 moves #3 (4+2)
[8/P7/8/5R2/4p3/8/2K1kB2/8]

Try: {1.a8=Q? Kf1!}
Key: 1.a8=B!
1...Kf1 2.Be4 Ke2 3.Bd3#
1...e3 2.Bg2 exg2 3.Re5#

	(Problem 31) Sam Loyd, ”Holyoke Transcript”, 1876 White plays and mates in 3 moves #3 (6+3)
[ b7/PP6/8/8/7K/6B1/6S1/4R1bk]

Try: {1.bxa8=Q? (Black is in stalemate! No legal move exists for black, and that means white has lost the opportunity to win)}.
Key: 1.bxa8=S! Kxg2 2.Sb6 ~ 3.a8=B/Q#

Comments: Here the key takes a piece (we today believe this is not a good first move for an orthodox problem), sacrificing at the same time Sg2 (and the composer believes that this brings equilibrium).
Anyway, the key presents a clever underpromotion: The white knight Sa8 is needed to protect pawn a7, which will be threatened from the black bishop Bg1, just when bishop is unpinned.
The composer believes that is not a defect, that in the last move of the solution the promoted piece can be B or Q.

	(Problem 133) Endre Ancsin, 3rd Prize, ”Internationaler Mannschaftswettkampf”, 1936/37 White plays and mates in 3 moves #3 (13+13)
[2ss3B/2SpPp1B/K4Qq1/1R4r1/pb2kpP1/1r3pP1/2PPbP2/4R3]

We see a white rook pinned by a black bishop, which is pinned by a white rook. We also see a white queen pinned by a black queen, which is pinned by a white bishop.

Try: {1.d3+? Rxd3!}
Try: {1.Rxe2+? fxe2!}
Try: {1.Bxg6+? Rxg6! (and the queen remains pinned)}.
Key: 1.e8=S! [2.Sd6+ Bxd6 / Sxd6 3.Qd4#]
If 1...Rd3 2.Rxb4+ Rd4+ 3.d3#
If 1...Se6 2.Qd4+ Sxd4+ 3.Sf6#

In the variations of the solution we observe cross-checks. The white gives check, the black answers with check, the white answers again with check.

[This post in Greek language].