In chess and other related games, a double check is a check delivered by two pieces simultaneously. In chess notation, it is almost always represented the same way as a single check ("+"), but is sometimes symbolized by "++". (The symbol "++", however, is also sometimes used to denote checkmate.) This article uses "++" for double check and "#" for checkmate.
Chess
The most common form of double check involves one piece moving to deliver check and revealing a discovered check at the same time from a piece it had been blocking. The only possible reply to a double check is a king move, as it is impossible to block or capture both checking pieces at once.
In exceptional circumstances, it is possible for the moved piece in a double check to not give check. The only way for this to happen in orthodox chess is by way of an en passant capture. In the position shown from Gundersen–Faul, 1928, Black has just played 14...g7–g5. White replies 15.hxg6 #. The result is a double check even though the white pawn does not give check: one check is given by the rook, discovered by the capturing pawn's move; the other by the bishop, revealed by the captured pawn's removal. Such a double check is extremely rare in practical play, but it is sometimes found in chess problems.
Game examples
A double check is a part of the smothered mate pattern known as Philidor's legacy.
Aron Nimzowitsch wrote, "Even the laziest king flees wildly in the face of a double check." Because the only possible response to a double check is a king move, the double check is often an important tactical motif.
A double check was also seen in the celebrated Evergreen Game, Anderssen–Dufresne, 1852.