Cross-checks, King flights, in the two-mover

Click on the 1.? after you have tried to solve it.

The main threat with the key is 2.Qxe4 mating, (the key paves the Queen's control on the e6 square initially obstructed). However, the main play is that the key sets up a surprising strong line of attack on the white-king, allowing the Black king to give as many as three discovered checks. These may be termed a royal battery, the King being the firing piece and the queen giving white check from the rear. For each, White's checkmating reply is separately forced as we shall see soon. This sequence where B gives W check, and W checkmates right back is called a cross-check. (As many as five cross-checks have been shown in a single problem, a "task".)

If 1...Ke6 ch, 2. Sc5 is dbl ch and mate. The knight blocks the discovered check of black, arriving on c5, it discovers double check from the light-sq bishop, and the king cannot move to the seventh rank, since the knight had also discovered the W Q's guard of e7!

If 1...Kg6 ch, 2.Se5 ch mates, (e5 interposes the check), g7 is not a run-out for His Majesty, since W Q's guard on g7 has been discovered, and B does not have QxS which is illegal(!) since Her Majesty is pinned upon this King flight by WR on g2.

If 1...Kxf4 ch, 2.Sd5 dbl ch mates (d5 interposes the check), it is dbl ch, discovered from the Rook battery down the f-file this time, and e3 square is diabolically covered by the checking Knight.

Slightly less thematic, but still cute, is the last variation 1...Qxf4, not a cross-check or a King flight. Now 2.Sb8 mates. The king cannot run to f4 on account of the "self-block", and the action to prevent the black rook line on the eighth rank is called a shut-off mate.