Checking key?

In orthodox problems (those which follow the rules of the usual chess game), if we manage to show with a checking-key a combination or a theme, which can not be demonstrated with a non-checking-key, then we accept the problem as quite proper. (That means, when we try to solve a problem, we do not rule out the possibility of a checking key).
An orthodox study may have a checking-key, but surely it is more impressive when it starts quietly.
In heterodox problems the checking-key is not unusual.

Theme : The key gives check.

	(Problem 26) N. G. G. van Dijk ”American Chess Bulletin”, 1961 White plays and mates in 2 moves #2 (6+1)
[3S4/2P5/B2k4/3S4/6K1/8/8/5Q2]

The key allows the black king to go to all his cross-shaped flights.

Try {1.Qb5? (black is in zugzwang) Ke5!}
Try {1.Qc4? Ke5!}
Try {1.Qf5? Kc5!}
Key: 1.Sf7+
1...Kd7 2.c8=Q#
1...Ke6 2.Qf5#
1...Kd5 2.Qc4#
1...Kc6 2.Qb5#
The black king has one more (diagonal) flight:
1...Kc5 2.Qc4#

Theme : Flights in shape of a cross (+ flights) : The black king may flee by moving on the row or on the file.

	(Problem 27) Skuja Roman, 1940 White plays and mates in 3 moves #3 (5+2)
[5K2/3P4/2k5/8/6B1/4B3/7Q/b7]

In the problem-27 we observe the method of the guidance. In two moves two queens are sacrificed, but this is not bothering us, since the goal is to mate the black king at the third move.

Tries {1.Qh1+? / Qg2+? Kb5!}
Try {1.Qb2? [2.Qb6+ Kd5 3.Qe6#] Bxb2!}
Try {1.Qc2+? Bc3!}
Try {1.Qd6+? Kxd6!}
Key: 1.Qc7+! (the sacrifice is guiding the black king)
If 1...Kxc7 2.d8=Q+ Kxd8 3.Bb6# (or 2...Kb7 / Kc6 3.Qc8# / Qd7#)
If 1...Kd5 2.d8=Q+ Ke4 3.Qf4#
If 1...Κb5 2.Qc5+ Ka4 / Ka6 3.Bd1# / Qb6#

	(Problem 28) Nenad Petrovic, “Schachmaty v SSSR“, 1960 White plays and mates in 4 moves #4 (4+2)
[8/1Q1K3s/1B6/1k6/8/8/1P6/8]

Try {1.Kd6? Sf6!}
Try {1.Bg1+? / Bf2+? / Be3+? / Bd4+? / Ba7+? Kc4!}
Try {1.Qd5+? Kxb6!}
Try {1.Qc6+? Kb4!}
Key: 1.Qa6+! (the sacrifice is guiding the black king)
If 1...Kxa6 2.Kc6 S~ 3.b4 S~ 4.b5#
If 1...Kb4 2.Qd3 Sf6+ 3.Kc6 S~ 4.Qb5#

	(Problem 29) M. Lipton, First Prize, ”Segal Memorial Tourney”, 1962 White plays and mates in 2 moves #2 (14+10)
[1b3ssq/1BP1p1PS/2p1k3/R1BS3P/8/1P2KP2/r1P1R1Pr/1b6]

In the prized problem-29, by Lipton, we see four tries with X-flights of the white Ke3, (the king opens the royal battery discovering Re2 which gives check), to be answered with X-flights of the black Ke6.

First phase, Try: {1.Kd2+? Kxd5! (pawn c2 is pinned, so 2.c4+ is impossible)}
Second phase, Try: {1.Kf2+? Kf5! (pawn g2 is pinned, so 2.g4] is impossible)}
Third phase, Try: {1.Kf4+? Kd7! (pawn c7 is pinned, so 2.c8=Q+ is impossible)}
Fourth phase, Try: {1.Kd4+? Kf7! (pawn g7 is pinned, so 2.gxf8=Q+ is impossible)}
Fifth phase, Key: 1.Kd3+!
1...Kxd5 2.Ba3# (pawn c2 is pinned differently, and the mate is changed)
1...Kf5 2.g4#
1...Kd7 2.c8=Q#
1...Kf7 2.gxf8=Q#

[This post in Greek language].