The Cover

The cover is a piece standing on an open file in front of the enemy heavy pieces: it alone prevents the opponent from seizing the line. Every trade offer around the covering piece is therefore loaded with consequences — whoever removes the cover takes the file. The examples, from Spassky–Karpov to Karpov's revenge against Unzicker, show both sides of the story.

. Cover! . White seize the a-file.