BLOCK

a problem in which the key provides no threat, but instead waits for Black to move and every move leads to a mate. In a complete block, all of the black’s moves have mates provided in the set play and the key is simply a waiting move; in an incomplete block, not all black moves are provided with mates in the set play – the key provides for those that don’t; in a mutate some of the mates provided in the set play are changed following the key.