A discovered check is a tactical device in chess where moving one piece reveals an attack on the opponent’s king by another piece that was previously blocked behind it on the same rank, file, or diagonal.
The piece that moves (often called the "screen" or "cover" piece) is not the one delivering the check — it is the piece it uncovers that attacks the king. This creates a uniquely powerful threat: the moving piece is free to land on any square it chooses, whether to capture a piece, create a new threat, or occupy a strong square, while the opponent is forced to deal with the check from the revealed piece.
In practice, discovered checks are among the most dangerous tactics in chess because they impose two problems at once. The opponent must respond to the check, which usually means the cover piece’s action goes unanswered. Look for positions where one of your pieces is aligned with the enemy king along a rank, file, or diagonal, but is temporarily blocked by another friendly piece — moving that blocker can unleash a decisive discovered check.