We are interested in situations where Black can double check White in a two move problem. Since a King in double check must move, we see that such a double check by Black must be met by a royal battery. We will explore this situation in depth in this post. This was one of my first research experiences in in the theory of chess problems. We also refer readers to the July issue of StrateGems in 2016 (SG 75) for my article.
Here is the record for double checks by Black in a two mover by the task master himself. There are two set double checks 1...Rxc3++ 2.Kxc3, 1...Rxd4++ 2.Kxd4. The key gives up one of these mates and allows two more double checks:
There is some nice by play:
Of course there are several unprovided checks that detract.
The above problems give you a typical way for Black to double check in a two mover: Black fires a battery that leaves the rear battery piece pinned while the front piece is captured by the wK which at the same time fires its own battery. I set out to investigate the following question: is it possible for Black to double check with out the checking pieces being captured? The answer is yes and here are some examples.
One simple way to pull off the double check by Black without a capture is to use a R+P battery. The reason is that the pawn, while it provides check, cannot occupy the square just vacated by the wK. Here is an example of my own. Notice the set play: 1...d4+ 2.Ke4. The wQ sacrifice key makes a double threat:
Here is something spectacular, albeit heavy, from the great Italian composer. Notice the masked Black battery aimed at the wK on f7. The set check 1...Sxd6 is met by 2.Kxe7. The key changes this and allows five (!) checks on the wK including a double check:
Here the double check is executed with a R+S battery and the bS must self pin when performing the check.
Here is one of my personal favorites. This time the wQ unpins the wSe5 allowing checks galore.
In my investigation I noticed that I could not find an example in which a bR and a bB delivered the double check without capture. The geometry will not allow for a R+B or B+R battery to be fired with the pinning that is required. However, there is another way! It turns out that it is possible for a bR and bB to double check without either piece moving!
The key to having a bR and bB double check and both be pinned is to use an en passant capture. Notice that the wK sits in check in the diagram. This is necessary: it will be impossible for a bB and bR to double check and not be captured in a two move problem unless the wK is in check in the initial position.
1...cxd3 e.p.++ 2.Kxd3
Certainly, the key is obvious with the wK in check, but the play afterward is really nice with interference, self-blocking, and three wK battery mates.
One final open question remains. In the above problem the piece that fires the battery (the pawn) is captured. Is it possible to have a bR and bB double check and not capture the pawn? The answer is yes but I think it requires a checking key.
The wK moves f4 because it is the only square available after the en passant capture! Question: can any one pull off this task without a checking key? Specifically, is there a #2 with a double check from a bB and bR without capturing the bB, bR, or bP that does not have a checking key? Send me a comment.