Checking Keys

 
Checking keys are considered crude.  The great Comins Mansfield never composed a problem with a checking key.  Sometimes they are justified by great economy or surprise effects.  Here are some of my favorite checking key problems starting out with one of my own.

Moen, Kabe
Original

#2

4 + 2

Here is a very simple checking key problem.  The key 1.Rf5++ fires the B+R battery and does several things: gives a flight at e6, sacrifices the wR and gives a flight at f5, while also taking the unprovided flights at f7 and g7.  1...Kxf5 leads to the model mate (every square in the bK's field is only covered once and all white pieces participate) 2.Qg4.   A fun question: the problem is sound and has the same key with any black piece on e7: P,S,B,R,Q.  However, only one type of piece contains a dual.  Which one?

Loyd, Samuel
La Strategie 1867

#2

7 + 3

A classic Loyd.  Loyd said that this was his most difficult problem and that he stumped many friends with this.  I remember trying to solve this problem myself and being stumped.  Notice there are mates set for all moves of the bK.  The key abandons these mate with the striking 1.Qf4+!  It takes two of the flights but gives two more 1...Kd5/Ke5 2.Qe4, and it changes the mate after 1...Ke7 to 2.Bxd6 (this is the role of the wBa3).  But the most beautiful defense is by the bPf7 (which also had a set mate 2.Qxd6): 1...f5 2.gxf6 e.p.  That's right the en passant​ capture sets off a double battery.

Tomasevic, M.
Mat Plus 1996

#2

4 + 2

 
A good reason for a checking key is economy.  It does not get better than this 6 pieces and a star flight by the bK with nice play.  Key 1.Rb5+.

Loyd, Samual
New York Grachhic

#2

8 + 8

The great Sam Loyd had a few nice checking keyed problems (we have already seen two 1.Bd5+ and 1.Sg4+).  Here the key 1.Qe4+ moves the queen to a square where it can be captured by 5 pieces. 

Ua Tane
Good Companions Folder 1918

#2

12 + 6

Here is a checking key problem that Comins Mansfield praised in Adventures in Composition.  The key 1.Sc4+ grants five flights and leads to several promotion mates.  The composer seems like an interesting fellow.  His real given name is James Stimson but he changed his name to Ua Tane after he moved to the island of Tahiti.  

Ua Tane
Good Companions Folder 1919

#2

11 + 10

Here is another checking key from Ua Tane.  The key 1.Sc5+ sacrifices 4 pieces to the bK.

Vaughan, Colin V.
The Problemist Supplement 1999

#2

7 + 6

 
I always love to quote problems from the Problemist Supplement because it caters to beginning composers.  Here the key 1.Qc4+ sacrifices both wRs and allows for some nice battery play.

Bettmann, Henry Wald
The Problem 1914

#2

10 + 9

Here is a special checking key problem.  The position is a complete block: there are mates set for every black move.  However, there is no waiting move that holds the block.  It is not a mutate or a block threat either.  Instead it is a block check!  The key 1.Qh5+ unpins the bR.