A Double Check is the most powerful move in chess. It occurs when two pieces attack the enemy King at the same time.
When a king is in double check, the only way to escape is to move the king. You cannot block two attacks at once, and you cannot capture two pieces in one move!
This usually happens through a "discovered attack"—you move one piece to give check, and that move uncovers a second piece that also checks the king.
Because the king must move, double checks are incredibly dangerous and often lead to an immediate checkmate or the loss of a major piece.
Tactically, the double check is a forcing move that strips the opponent of all defensive options except flight. Since the King is the only piece allowed to move, any defenders that were ready to block a single check become completely useless. This often allows the attacking player to ignore the opponent's pieces entirely and focus solely on the King's escape squares.
Mastering the setup for a double check requires looking for "batteries" where two pieces are lined up toward the enemy King. By moving the front piece to a square where it delivers its own check, you unleash the hidden power of the piece behind it. This sudden surge of pressure can break even the strongest defensive positions in a single turn.