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Chess Problem 2003g4e501

BDS & Michael McDowell

2nd Prize, Probleemblad, 2003

8/1S2BB2/8/*3R3kS2/2*3B1p1K1/3rb3/4s3/8

#2
PAO a5:
VAO c4 

1.Sd4!  (2.Sc6#)

1...Sxd4 2.VAb5#
1...Rxd4 2.VAd5#
1...Bxd4 2.Sc5#
1...Kxd4 2.Bf6#

Following a flight-giving, sacrificial key, there are four variations capturing on d4. If a powerless black unit were to land on d4, three mates – 2.VAb5, 2.VAd5 and 2.Sc5 would all be possible, but only one of those mates is possible after each non-royal capture on d4. The fundamental error in these variations is self-block and the mates are separated by dual-avoidance. This amounts to Stocchi Dual Avoidance.

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