|In chess, a double check is a check delivered by two pieces at the same time.|
The most common form of one check involves one piece moving to deliver check, at the same time revealing a discovered check from a piece behind (such a check is an inherent part of the type of smothered mate known as Philidor´s legacy). The only replies to a double check are king moves, as capturing the checking piece (except by the king, which in the process moves out of check-by-the-other-piece) is not an option since there are two of them, and interposition is likewise impossible as there would be two lines of attack to block.
Exceptionally, the piece moved does not give check. The only way for it to happen in orthodox chess is by way of an en passant pawn capture. In the position shown to the right, Black has just played g7-g5 (see algebraic notation); white replies hxg6 e.p.+. This is a double check, yet the pawn moved does not give check: one check is given by the rook, the other by the bishop; the former is discovered by the movement of the capturing pawn, the latter by that of the captured pawn. Such a double check is extremely rare in practical play, but is sometimes found in problems.
In chess with variant rules or fairy pieces, other ways of delivering a double check may be possible. Triple checks may also be possible, for example: White rook e1, bishop e4, moas d3 and f3, king h1; Black king e5; White plays 1.Ba8 triple check (the moa is a non-leaping knight which first takes a diagonal step, then an orthogonal one).