Explanation

A double check is a check given by two pieces simultaneously. One piece moves and gives check, and also discovers a check from another piece.

It is more difficult for the opponent to defend against a double check than it is to defend against a normal check. There are three ways to defend against a normal check: Capturing the piece that gives check, blocking the check, and escaping with the king to a safe square. Against a double check, it is impossible to capture both pieces giving check with one move. Blocking the two checks with one move is also impossible. Therefore, the only way to defend against a double check is to escape with the king.

If there are no safe escape squares, the double check is also checkmate, a double check mate.

Examples

White can deliver double check mate with Nxf6#.
Black is unable to defend, even though both pieces giving check are under attack. Black cannot capture the queen with Bxe2, nor the knight with Qxf6 or gxf6.
The knight also guards the escape square d7.

White can deliver double check mate with Rd8#.
Black cannot capture the rook with Nxd8 or Bxd8.
Rook and bishop are guarding all escape squares.

Pattern matching

If you can give a discovered check, and also give a check with the discovering piece, it is a double check.
Always look closely at all double checks, as the only defence is to escape with the king.
Even if it is not checkmate immediately, you might be able to checkmate the king or win material anyway.

Related patterns

In discovered checkmate, the moved piece does not give a check but discovers a checkmate anyway.
In double check into checkmate, a double check is followed up with a later checkmate.