The key is the first move of the solution of a chess composition. The key is a move by the white in all kinds of problems (Orthodox problems, Selfmate problems, Studies, etc.), except in the Helpmate problems where the key is a move by the black.
The key must be a move which can be found with difficulty, in order to justify the determination “chess problem”.
The key must be the unique move that solves the problem. If there is other move, (which the composer is not aware of), which move also solves the problem, then the problem is considered useless and is called cooked. The exception here is those problems that deliberately have more than one solution, as it is usual in Helpmate, where one solution complements the other in a prominent way (solutions with same strategic, omostrategic solutions).
In all types of problems, as key is allowed any normal move or taking of piece or promotion of pawn, but specially:
(1) castling is allowed (king’s side castling 0-0 or queen’s side castling 0-0-0), except if it can be shown with Proof analysis that the king or the relevant rook has made a move, (thus castling is not allowed).
(2) en passant (e.p.) pawn taking is not allowed, except if it can be shown with Proof analysis that this pawn has moved with two steps exactly in the previous move.
We will see soon when a key is considered good or not, by examining some characteristics of the keys.
[This Post in Greek language]