Types of Checks
A simple and very common type of check is when a piece moves to directly attack the opposing king only by itself. Sometimes such a check is part of a chess tactic such as a fork, a skewer, or a discovered attack on another piece. In some cases, a check can be used to defend against such tactics.
There are also a few more special types of check:
* Discovered check - A discovered check is similar to any other type of discovered attack except that it is a discovered attack on the opposing king. In a discovered check, a piece moves out of the line of attack by another piece so that this other piece (which can be a queen, rook, or bishop) is then checking the opponent's king. The piece that actually moved in the discovered check move could possibly be any type of piece belonging to the same player as the checking piece except queen or the same type of piece administering the check. A discovered check could be a tactic in itself because the piece that moved could attack or otherwise create a threat to another piece on the checked king's side. The opponent has to get out of the discovered check on the following move and may not get a chance to thwart the attack by the other piece that moved.
* Double check - A double check is when two pieces check the opponent's king on the same checking move. This happens when the piece that moved during the checking move attacks the king and another piece checks by discovered check on the same move. It can also happen, though very rarely, when an en passant capture opens two lines of attack simultaneously. In algebraic chess notation, a double check move is sometimes noted with a ++ after the written move in place of the usual +, although sometimes ++ has been used to indicate checkmate in more modern literature (see here). When ++ is used to indicate double check, # is used to indicate checkmate, but using ++ for double check is obsolete.
* Cross-check - In chess, when a check is answered by a check, particularly when this second check is delivered by a piece blocking the first, it is termed a cross-check.
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