Characteristic 5 : The key closes lines of white pieces or opens lines of black pieces.
Characteristic 6 : The key gives flights to the black king.
It is also acceptable a key, called give and take, which deprives the black king from some flights but gives at the same time equal number of other flights. The flights may be empty unguarded squares or squares occupied by white pieces, which can be captured by the black king.
"Chess Record", 1876
White plays and mates in 2 moves
In the initial position of the problem the black king has no flights around him.
Key: 1.Sf2! (threat 2.Sxh3#. The key has characteristic 5, (because it closes the lines of the white pieces), and 6, (because it gives flights). It creates four flights for the black king, sacrificing additionally the knight Sf6).
If 1...Kh4 2.Se4#
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