Expert: Tony D'Aloisio - 5/26/2009

Question
my opponent failed to call check, i moved, it is now his move, what can he do, or does my move have to be reversed. i moved another piece, and the king is still in check.

Answer
It is difficult to answer such a question comprehensively, because the situation where the game is being played is crucial to the response.

If the game is played in an officially rated tournament (under the auspices of the United States Chess Federation, for example) the following considerations apply:

1) Players seldom will announce check (and stronger players as a rule never do).

2) If it is found that any illegal position has occurred (which includes a king not moving out of check), the position is returned to the position immediately before the illegality occurred, and play resumes.

3) Since both players are required to write down their moves on score sheets, the position before the illegal situation should able to be be re-created with no problem when the tournament director refers to one (or both) of the players' score sheets.

In a casual or offhand game though, the rules are not so stringent.  You should probably try to return to the original position where you are in check, and make your move from that situation.  Again though, if the position was several moves ago, and no one has been writing down the moves...that could prove problematic.

One additional note:  it is the custom in blitz games (i.e., speed chess, where each side has only a few minutes to make all their moves) that, if a king is left in check, the opponent may capture it and thus end the game immediately.  Note that this option ONLY applies in speed chess.