Double check

In chess, 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 "++" (however, "++" is also sometimes used to denote checkmate).

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. (Such a check is an inherent part of the type of smothered mate known as Philidor's legacy.) The only possible replies to a double check are king moves, as both checking pieces cannot be captured or blocked at once. In some cases, the king can capture one checking piece and escape the other in a single move.

In exceptional circumstances, it is possible for the moved piece to not participate in the double check (i.e. to give a "double discovered check", though this name is not actually used). The only way for this to happen in orthodox chess is by way of an en passant capture. In the position shown at right, Black has just played 1...g7–g5. White replies 2.hxg6e.p.++. The result is a double check even though the pawn White moved does not give check (one check is given by the rook, discovered by the capturing pawn's move; the other by the bishop, discovered by the captured pawn's removal). Such a double check is extremely rare in practical play, but is sometimes found in problems.

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 famous example is Réti–Tartakower, Vienna 1910, which arose after 1.e4 c6 2.d4 d5 3.Nc3 dxe4 4.Nxe4 Nf6 5.Qd3 e5?! 6.dxe5 Qa5+ 7.Bd2 Qxe5 8.0-0-0! Nxe4?? 9.Qd8+!! (sacrificing a queen in order to set up a double check) Kxd8 10.Bg5++ and White mates after 10...Ke8 11.Rd8# or 10...Kc7 11.Bd8#.

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