A double check is a special discovered check where the moving piece and the piece it unveils both give check at the same instant.
It happens when you move a piece to deliver check while simultaneously uncovering a second checker behind it. Now two of your pieces attack the king at once.
This is the most forcing move in chess: you can’t block two checks on different lines, and you can’t capture both checkers in one move, so the king is absolutely forced to move. That makes double checks the backbone of many king hunts and smothered mates.
Because the king must step away, a double check often works even when you’re sacrificing material — the opponent never gets the chance to take your pieces.
Because the two checks come from two different pieces on two different lines. Blocking one still leaves the other, and you can’t capture both checkers in a single move — so the king has to move.
A double check is a type of discovered check where the moving piece also gives check, so you end up with two checkers instead of one.
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