”N.Y. State Chess Association”, 1892
White plays and mates in 2 moves
In the initial position there is the battery Bf5-Rf6. When the front piece of the battery moves, the second piece is discovered and threatens the king.
Key: 1.Ra6! (Destroys the battery of bishop Bf5 – and rook Rf6, Bf5 remains unprotected and threatened by four black pieces, and the black king has got a new flight. But now Be7 guards the square g5 and there exist the threat 2.Qf1#).
In the variation 1...Kxf5 2.Rf6# we have switchback of the rook.
In the variation 1...Rxf5 2.Rxa4# becomes obvious the reasoning behind the key-move Ra6.
Threat or Wait?
Usually the key introduces a threat.
Let us suppose that white plays something but does not make any threat. If black does not play (it is black’s turn to play) then black is not in danger! Unfortunately black is obliged to play.
If black, playing anything, makes his position worse, then we say that black is in a zugzwang situation (zz), thus they are forced to lose.
If black is almost in zugzwang, then the key may bring black exactly in this situation. (See problem-34).
If black is already in zugzwang, the white must make one waiting move as key, in order to come black’s turn to play and lose.
In some of the problems this waiting move is difficult to be spotted, because every white move seems that it spoils set mates. (See characteristic 10).
Characteristic 8 : The key brings in zugzwang the opponent, or sustains this zz situation.
[This Post in Greek language]