Double Check

Double check names itself plainly — two checks delivered at once — and Howard Staunton's 1876 Chess: Theory and Practice defines it as occurring "when both the man moved and the one unmasked attack the King." It is the most forcing category in the discovered-check family precisely because the two standard defenses to check, blocking the attacking line and capturing the attacker, cannot resolve two separate checking pieces simultaneously; a legal king move is essentially always required. Because it combines a discovery with an unavoidable, often decisive king move, double check frequently appears at the climax of famous mating combinations, where the forced king move walks directly into a prepared final blow.